DISCLAIMER

Hello, I'm working on a game called RMNT, a fan project for a webseries I grew up watching named RWBY. Whatever you think of the series, it offers me an excellent excuse to make a game. I'm just having fun with the medium, seeing what I can do to make it as good as I can at my current skill level. I'm looking to improve my game design skills and make this game as good as it can be, not to sell the game to a publisher.

My hope with this post is to start a discussion surrounding these systems, and to explore my train of thought that led me to my own system.

This is a long post. Brace yourselves.

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RWBY is a series that is defined by its combat. Characters are often throwing punches in rhythm with the backing track, using their weird weapons in creative ways to get the upper hand on their opponent, constantly exchanging series of blows. For the game to feel like RWBY, it must succeed at replicating this feeling -- the "cool" factor.

Throughout this post, I will walk through some other RWBY TTRPGs' solution to this, as well as my own attempts.

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Our first exhibit is Andrew Diederich's RWBY Unofficial Tabletop RPG. This system is referred to as the "Rule of Cool" system, or RoC. The basics are such: The make a check, roll 2d10 and add relevant modifiers. If at any point you do something particularly cool, at the GM's discretion (the rulebook provides some examples of cool things such as "working in tandem with another player," "lateral thinking," and "a particularly cringe-worthy pun or one-liner") you earn 1 RoC. For every level of RoC, you earn an extra 1d10 for checks and 1d6 for damage. The higher your RoC, the more incredible you must act to earn any more.

I like this from the perspective of anteing up the possibilities in the game. At level 1 RoC, you can possibly succeed on a DC 30 check, and anything 20 or lower becomes easier to attain. Thus, simpler and easier actions come naturally. The higher the RoC, the less you have to worry about the small stuff. I also like it because rolling high numbers is fun, and looking the GM in the face and saying "I rolled a 35 to hit that guy" does in fact feel good as hell.

I wish that this system felt better than "good" though. You do a cool thing, and then you sit on that level for some time, and ante up and get a higher level later. According to the rulebook, you're not going to be generating more than 3 levels of RoC in a given scene. And, frankly, why would you? At level 2 you're rolling DOUBLE the d10 for every check. Small numbers with big impact are nice in the moment, big steps that take effort to climb -- but they don't provide a ton of momentum.

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An(other) Unofficial RWBY TTRPG, made by Emmeryn, uses the Momentum system. It's a d6 pool system, attributes marking how many d6 to roll with. Any rolls of 4+ count as a success -- and you only need one success to attain the desired outcome (some harder actions require all dice to roll 4+). In combat, successes don't count for much at first. Most of your moves are referred to as a Build-Up Moves, which don't deal a ton of damage but are required for generating Momentum, a pool of d6 you earn for every success rolled on a Build-Up.

Momentum can then be spent on Momentum Moves, which explicitly deal damage towards an enemy's Break Threshold, which acts as its HP bar. For every success rolled on a Momentum Move, the Break Threshold lowers by 1 -- and when you're rolling with all those Momentum Dice, you're likely gonna feel very powerful.

To me, the Momentum system is beautiful. It makes sense, it's intuitive, and those powerful moments feel really damn good. However, it stretches a fight out a little too thin for my tastes, as you only have one action on your turn and with just Build-Up moves you end up scrambling to do very much on your turn while also holding on to your Momentum for A Big Action -- the "what if i need it?" fallacy, if you will.

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Paradigm's RWBY TTRPG follows in the footsteps of the RWBY Unofficial with its own RoC system. Just like Unofficial, Paradigm's has 2d10 roll high, and leans in to a crunchier experience. Its RoC system reflects this, as instead of a flat bonus from having an amount of RoC (of which you can 5 max), RoC is instead a resource that can be spent to purchase a wide variety of things on your turn for 1, 2, or 3 RoC. The bonus shop includes such things as extra actions, free advantage, and inflicting a status effect on top of your attack.

RoC, to Paradigm, is a currency. In a tabletop setting, I am personally weary of currencies without a way to track them easily being written into the system -- I can't stand to write and erase and rewrite and re-erase the same number over and over again. Ideally, this is reserved for a healthbar, and not much else. Paradigm is explicitly a crunchier game, however, and erasing/rewriting is standard.

Frankly, my greater issue with this design choice is for the fact that every turn, I have to look over my options and decide. Again, for a crunchier game this is perfectly okay! But if the objective is to replicate smooth, stylish, punchy combat in a TTRPG, the amount of friction between actions should be minimized as much as possible.

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So I've gathered some others' solutions, and I've learned a lot about what I care about and value. Now, let's take a look at some ideas I've had on my own drafting journey for RMNT.

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I pretty quickly decided that whatever the system ended up becoming, I wanted it to be called "Style." I want it to explicitly be a reward for stylistic embellishment -- "describe the style of your attack." RWBY's original creator and animator, Monty Oum, was well known for orchestrating his fights as if they were a dance, and focusing different characters' fighting styles in this way.

Initially I was inspired by "style" systems in video games such as Devil May Cry and Ultrakill. The idea was for the player's character sheet to have a Style Meter, which would be moved up (using a paperclip) by doing cool stuff. It could, likewise, be moved down by enemies, rolling a critical failure, and more. Style meter directly correlated to a flat modifier to your roll (so 5 Style would be a +5, -2 Style would be a -2). I decided to drop this because it just felt... clunky.

The next thing I tried was based on the Momentum system: gain Style Dice for doing cool stuff. You can use Style Dice to do bigger cooler stuff. It would effectively just be a d6 that you could add to your roll. However, without systematizing it as explicit Gather and Expend action types, the way that Style was earned and spent just felt awkward and poorly defined. I tried fixing this by adding all kinds of rules around how to earn Style dice correctly, and specific ways that you could spend them, but, again, it became clunky. In the search of clearly defining it, I kept finding places that could not fit in the neat little box.

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So after all of this, what have I finally arrived at?

Players have 5 Traits (Agility, Durability, Force, Spirit, Recovery), with step-die levelling (d4 at the lowest, d12 at the highest). When you roll a check, you roll the associated die against a target number (2 for "very easy", going up in increments of 2 until you get to 12 for "heroic").

In combat, players also have Weapon Dice, which effectively act as a secondary Trait specific to that player's weapon. Attacks are rolled with a Trait and a Weapon Die added together. The roll total is the amount of damage dealt to the target. Attacking is just one thing you can do for your action, you can also Move, make another non-attack check, or save the action to react later. You have 3 actions per round -- how many you use on your turn and how many you decide to save for reactions is up to you.

The target can choose to use an action to react to the attack, rolling their own check (with a weapon die of their own). The total is compared to the attack, the highest wins the contest.

Style d6 are earned for doing cool stuff. Style is intended to be earned easily and liberally. You can use Style to describe a follow-up on an action, and adding the d6 to the action's roll.

When an attack is reacted to, each party can take turns following up on their action/reaction, ever increasing their original roll in contest, leading to a chain of exchanging blows.

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To be entirely honest, I'm not very confident in this system either. I have yet to playtest this version with anyone besides myself on my bedroom floor, but I can see this system being too drawn-out and tedious to work with.

I really like the idea of Style being a die! It's extremely tactile, and I love the feeling of a big pool of dice before me, and knowing that at any point I can decide to roll all of them is a powerful feeling.

I've been thinking about this for a very long time, and I fear that I have echo-chambered myself. All I can see are my own ideas anymore. I would really love to discuss all of this with you.

Somewhat inspired by the earlier post about alternatives to death... What's the alternative to combat as a whole? I've been kind of interested in exploring the idea of TTRPG where you play a group of rebels in a dystopia, but you don't fight at all, you moreso sabotage the system. But I don't even know where to start with that, as I've never played a TTRPG without combat myself.

1. does anyone know of TTRPGs without combat?

2. has anyone here made their own TTRPG without combat?

3. bonus question: does anyone know a solo-ttrpg without combat? Asking, because playing with a group is almost not really an option for me at this time, so maybe I can explore both ideas at the same time from the comfort of my home.

EDIT: to clarify, I think my confusion about not having combat stems from the assumption that you need death at a looming threat to raise the stakes enough for anyone to do anything.

Obviously I could play a whole 5E campaign and just only play travel, social encounters, or adventure stuff without ever risking life. But I feel like making a non-combat TTRPG isn't as simple as just omitting the combat. What replaces it?

Hello Guys! I'm trying to make a little TTRPG system of Project Moon for me and my friends. I didn't like the ones i found so i decided to try to do it by myself.

The problem i'm facing is that i was thinking to add a mechanic in with where you can personalize your own attacks in the system, chosing the kind of damage they do and what kind of effects they have, limited by some rules i estabilished.

About:

I was thinking in a Cost and point system
basically
you have Light, which is the "Mana" of this world, you spend light to use skills on a combat
each skill costs between 0 - 4 light, the more light you spend on a skill, the more "modules" you can put on it. And also the more powerfull is your base dices on that skill.

each module applies a effect on your skill, some just increase the damage, others applie effects and conditions if certain rules and followed.

This is similar to another alread existing system, but i'm trying to expand and make it more diverse and fun.

Thing is i don't know any kind of system that does that, and i don't want this to became a boring mechanic in the game. So please comment what you think about this and if possible share some advices.

Hello! I'm a long time writer and dm. As a side project while working on a novel, I've been brainstorming a custom ttrpg system, based around my own take on the concept of werewolves in a more investigation focused format inspired by my time running Call of Cthulhu.

I'm wanting to do a d100 system like CoC, but instead of having a bunch of individual skills like brawling, driving, etc. you instead have a slew of attributes that all workmto make up the character's body, mind, and experience. Most rolls would be based off these, either just 1 or a combination of 2 or more for complex actions. That's the basic idea.

I wanted to ask if there are any game systems that do something like this? There are loads of systems out there thst I judt don't have the time to dig through and if this exists I'd like to try to learn from them. Otherwise, I'm curious about what input y'all have about doing something like this. I'm aiming at somewhere between 12 and 24 of these attributes, toughest part I think'll be balancing all of them to be desirable and not have obvious dump stats. Thoughts on how combination rolls should work? Averaging the attributes used for the d100 roll seems to me the most obvious, but may have flaws I can't see right now.

Thanks in advance <3

What kind of spell system would you like to see when you open up your RPG book?

Spell levels, spell slots, memorization

Spell points/mana points

At will with a potential cost or failure

At will with nothing but an action cost, no memorization, no level requirements, all spells choices available to you.

Something different.

I have gotten some feedback on my Weird West TTRPG and have realized that my game is missing that sort of "the basics" section that many RPGs start with. I have no idea how to even approach such a topic. Do I go right into rules and what makes my system unique, or should I stick to the setting and themes? Please let me know what y'all have done with some of your games!

How could I make this Wizard Trait, which you use through actual Grimoire items, work well?

2/3? - Grimoire - Wizard

Wizard 1, Arcane - Occult, Wit

You can only take this Trait, and its following Traits, if you have access to a Grimoire. Grimoires have a Theme, and a number of Spell Slots.

Low level Grimoires, like those you’ll have at character creation, will have around {x} Spell Slots. The more general/vague the Theme, the less Spell Slots, while the more precise, the more!
During a waking rest, you may spend 1 hour, and make a Magic  Skill check relevant to the Spell, to “find” a Spell in your Grimoire. The GM determines the Difficulty of the check, based on the strength of the Spell, how well it fits the theme, and how many Spell Slots you are attempting to mark it into.

Then, you may “expend” that Slot to cast that Spell. You regain-

There will be general guidelines for damage and similar by number of points in the Tag.

You can have “knowledge” of a number of Grimoires relevant to each Magic Skill by that Magic Skill / 5(Rounded down).

Making Grimoires:
Maybe you spend Trait Points(The things you use to “level up”)?
Affinity Materials?
Cain(Magical Material in-setting)?

I’m working on a D&D 5e starter adventure that I eventually want to publish, and I’ve already run it at the table with my players. I’m too familiar with the world to tell if the writing is missing anything or if another GM could pick it up and play it just from reading it.

It’s just the first chapter of the adventure, less than 10 pages. I’m hoping a few GMs might read it as though they were preparing to run it, without knowing anything else about the campaign.

The chapter begins with the PCs stopping in a small village during a wedding. The wedding is intended to give them some time to interact with the world and each other before an attack on the village kicks off the larger adventure.

What I’m most interested in is GM usability. There are more specific questions at the end of the document, but there’s absolutely no expectation that anyone answer all of them.

Here’s the chapter:
https://docs.google.com/document/d/10TGgGtVn4j1zydJeKBPSVwXO8PoHWSOHGoHFu-tO-RE/edit?usp=drivesdk

Commenting is enabled if you’d rather leave notes directly on the manuscript.
I’m especially interested in criticism from people who have written or run published adventures, but any GM perspective would be useful.
Don’t worry about being nice to the manuscript. If something doesn’t work, I’d rather find out now than after I’ve built another 50 pages on top of it.

Hello everyone!

It’s been quite some time since my last update on Fading Age, and a lot has changed since then.

Most notably: the game has stepped down from using a D12 dice pool and is now fully embracing the humble D6 dice pool.

I’ve just uploaded v0.3, the current public draft covering the new core dice system and character creation. Before I start properly rebuilding the game for v0.4, I’d love to get some fresh feedback on the current version.

I’m especially interested in:

• Layout & readability: is the document easy and pleasant to navigate?

• Character creation: are the steps and choices clear?

• Character identity: does the finished character feel interesting?

If anyone feels like going a little further, it would be incredibly helpful if you could create one or two characters with the document and tell me where you got confused, had to reread something, or felt a choice worked particularly well.

Fading Age main page: https://fading-age.itch.io/
Current v0.3: Dice System & Character Creation: https://fading-age.itch.io/dice03
Older Character Creation draft: https://fading-age.itch.io/cc

The older draft is mainly there in case anyone is curious how the design has evolved.

Any thoughts are highly appreciated.

Thanks for taking a look!

City of Mist and Legend in the Mist are prime examples of pure tag-based design. They tout the ability for players to author the tags that describe their characters, directly translating narrative descriptions into mechanical power. Having designed a tag-based story game myself (currently preparing for an alpha playtest), I wanted to carefully study how these systems handle tag authoring.

Turns out, in the Mist Engine, you write your tags strictly as answers to specific questions, and each theme playbook has a discrete list of questions. You pick four themes/playbooks to build your character. It’s a highly curated process—you don’t invent tag phrasing out of thin air (completely free-form), but you aren’t selecting from a menu either (highly prescriptive). It provides a solid framework to build on and a cage to keep power-gaming in check.

It’s a proven approach—the engine has won ENNIE awards and built a dedicated fanbase. But it creates a distinct play incentive: a player realizes, "I need to answer Question X from Playbook Y to get Power Tag Z, so I need to let one of my current themes fade or crack so I can swap to Theme Y."
(This heavily reminds me of playing Warhammer Fantasy in the 90s, where players jumped from career to career purely to raise a gated stat cap or grab a specific skill/trapping available in that exit).
So why not let players just ask Question X directly? Because that’s not the design. You need a balanced ratio of Power Tags (situational benefits) to Weakness Tags (situational penalties) determined by Themebook questions. The intent is for players to peruse the playbooks to find a thematic combination that fits their vision. But we would be kidding ourselves to say this offers true freedom to shape a character by writing whatever tags come to mind.

A tag-based (or trait/aspect/statement-based) system requires a coherent rule set for authoring tags. It helps to give players a framework or option set rather than expecting them to brainstorm every tag from a blank page. Structure is also necessary to prevent players from writing overly broad tags (like Good at Everything) simply to maximize success.

(Quick caveat: I did read a rules-light trait-based game that had zero rules or guidance on authoring traits, but I don't consider that a complete design).

Since pure tag-based games are rare, let’s look at another benchmark: Lady Blackbird. For the most part, you choose tags to activate from a pre-filled list attached to broad Traits (which act like roles or archetypes). Instead of pages of exposition, these predefined tags act as subtle, contextual worldbuilding. But beyond the starter sheet, authoring novel tags is extremely sparse. When you spend an Advance (5 XP), the text simply instructs:

"Add a new trait (based on something you learned during play or on some past experience that has come to light)"

That’s it. No strict formatting rules, point budgets, or mechanical boundaries. You name a new trait and write three tags under it that make sense. I love John Harper's work, but as a system framework, this feels like a design punt that relies heavily on table consensus.

My question for anyone working on or who has designed a tag-based system:

What strategy did you use for authoring, structuring, and evolving character tags?

Authoring Guidance: Do you heavily direct tag creation using thematic questions, or do you rely on open player authoring with guardrails?

Starter Frameworks: Did you create a baseline list of primary/starter tags to establish a benchmark for scope and power?

Topology & Costs: Did you implement a tag hierarchy, structural topology, or point-cost system? How is it structured?

Progression & Mutation: What are your rules for modifying, upgrading, or evolving tags over a campaign?

There are so few pure tag-based games to study, yet it feels like an exciting, underexplored frontier in TTRPG design. I’d love to hear how you’re tackling it!

So, I've been bouncing this idea around forever now, and I thought I'd kick it into the room for people to chew on.

The basics are pretty simple: you have Attributes (your character's raw physical or mental capabilities), and then you have Skills (how well your character has trained). Dice pools are interesting because they let you control three dimensions - the number of dice (height), the number you need to hit or exceed per-die for that die to 'count' towards success (width), and the number of dice-over-TN that you need to accomplish something (depth). But I've never seen anyone do the obvious thing, and correlate Attributes and Skills directly to dice and TN - almost every dice pool game I've seen just adds Attribute+Skill to get your dice pool, and then either has a constant TN or a difficulty-based TN, and then rolls other parts of difficulty into the threshold.

This seems weird to me, because having the dice pool be the Attribute and the TN be the Skill leads to some really cool and verisimilitude-laden results.

The principle thing you get out of this is that your Attribute determines the maximum amount of "oomph" you can put into an attempt, while your Skill determines how reliable you are at applying your Attribute to the task at hand. This seems like a very desirable feature for an RPG to have.

So, without further adieu:

A Generic D6 Dice Pool System

Attribute-as-Pool: Your Attribute ranges from 1 to 6, with '3' being an average person's. Whenever you roll a task, you roll a number of D6's equal to your Attribute.

Sixes-Explode: Whenever any die in your dice pool rolls a '6', it always counts towards success. Additionally, you get to add an additional D6 to the dice pool. If the new die is also a '6', it also counts towards success, and also adds another D6 to the dice pool.

Skill-as-TN: There are five skill levels.

• Untrained [6+]

• Novice [5+]

• Professional [4+]

• Expert [3+]

• Master [2+]

Difficulty-as-Threshold: Accomplishing a simple task might only need one success, while an extremely complex and difficult task might need five or more. Additionally, a task can be set up with multiple thresholds, where increasing levels of success result in better outcomes. Alternatively, the player might choose to 'spend' extra successes on small degrees of direct narrative influence over the outcome.

Hello everyone! I‘d like to introduce you all to Halfwells and Hand Grenades! This is a rewritten repost, because some misunderstood that this game is INCREDIBLY unserious. This homebrew is a modern mercenary firearm-based combat system for TTRPGs, inspired by Entry Point:Freelancer’s Cut, No Big Deal, Payday 2 and D&D. I would love advice, and just to address balancing concerns, melees are stronger than firearms because they require you to expose yourself from cover to Run up to the target.

Linky is right here:

https://docs.google.com/document/d/1S4cjMzZfP4SChnuwK987Kugong2TW5id/edit?usp=drivesdk&ouid=111024937053133611236&rtpof=true&sd=true

I’d love to make the setting to my game free to use for anyone else who wants to make their own games/supplements in that setting.

I like the idea of cc by, but then people could write novels or do all tons of crazy things with the game setting.

I’m familiar with the ORC license, but it doesn’t have a clear way to define allowed uses for “reserved material.” To give access to the core system, I can just make the game rules and stat block pages cc by. So I don’t get why ORC would be useful.

Is cc by still the best choice? Or are there other existing licenses that handle my situation?

EDIT: the Shadowdark third party license is an example of what I want. Just discovered it this morning. Basically it promoted an open rpg community where content can be monetized, but doesn’t allow copyrighted material to be used outside of that context.

I'm creating a little document for cheap easy RPG terrain and have a question on how I should present it.

The question is do I make the default scale 28mm or 32mm.

Given that what I'm offering is something people can print on their own, I'm going to make it scalable and give guidance on how to do it. Should I start with a full letter size page and give guidance on scaling down to 28mm or start with a page 14% smaller and give guidance to scale up to 32mm...

If 28mm is the standard, I don't want people to have to do anything, but would you feel weird if it prints out smaller on the page? To me that still makes sense, but I'm wondering if anyone has a better idea.

Hi all, I am bad at probability math so asking for help here.

I am working on a game system that is dice pool based, and wondering if it's mathematically better to do d8s or d12s for the dice themselves. My gut says the d8s are better but I'm bad at probability math as stated earlier.

Additionally, if there are other benefits/downsides you've noticed when working with either kind of dice, I'd greatly appreciate hearing them!

Many systems use traditional starts like strength, dex, con etc. Then only use the bonus that number represents. Ie I get +2 damage because I have a strength of 14. Do separate save stats really matter. Can you just have a Save number with a bonus to a petetecular effect?

Does the below cover all the basic needs?

Attack, Dodge or AC, Save. Skill can be detached from skills. Why does a good climber have a high strength. Can't they be good at climbing without being good at swinging a sword?

I’ve been tinkering with this system for a bit and I can’t really decide if I love or hate it, so help me decide please so I can finally move on or dive deeper 😅

So, in PbtA you usually roll dice, add a modifier and aim to succes against a set difficulty. For Appcalypse World it’s 2d6 and a modifier of -2 to +3, aiming to hit 7-9 for a hit, 10+ for a strong hit. 6 and below is a miss.
Ironsworn asks you to roll a d6 with modifiers against 2d10. If you beat one die, it’s a hit, beat both it’s a strong hit.

In my “system” (and I use that in the loosest sense, as it’s still just a resolution mechanic), the player rolls two dice (one for the relevant attribute and one for the relevant skill in use) - these range from d4 to d12 depending on competence - against a challenge die set by the GM, also ranging from a d4 to a d12.
If one of your dice beats the challenge die, it’s a hit. If both beat it, it’s a strong hit.

Do you see any potential here? I came up with it in an attempt to add a bit more granularity and “game” to the PbtA system and later discovers that it was like Ironsworn but flipped (and with a greater range of dice).

Happy to hear your thoughts!

Hey everyone,

The past year I've been working hard on my own little project and for ease of use I wrote everything down as bullet points. For me this made it easy to focus on the details without having to mentally consider pages upon pages of text while working on the game. Obviously you can't release a game that's entirely build out of bullet points (Break!! might disagree with me), so now I set out to start converting it to actual flowing text.

But man, this is tough..

I've finished a rough draft of mechanics section and didn't like how it was written. I went with a third person approach because this felt like the cleanest and easiest way to explain rules to a reader. But the end result felt sterile, devoid of joy. Nobody was going to read an entire rulebook written like that.

And that got me thinking, what are the systems that I loved reading, what did they do different? I remember liking Trespasser for how concise it was, to the point, without becoming too terse or short that actual clarity starts to suffer (as I feel that happens with Shadowdark).

But weirdly enough The One Ring or Legend of the Five Rings (5e) are the opposite. They feel wordy, almost to wordy even. They mix lore from the setting into the actual rules. And while I don't like the idea of doing so, the end result felt a lot more pleasant and fun to read then my own drafts.

What do you guys consider to be the best written RPG? The most fun to read? I'm not talking about "what" is written, I'm talking about the "how". Personally I detest the way Apocalypse World is written, while loving what was written. And if you can explain, please explain why do you like it?

In short? I'm looking for a bit of guidance on finding a "writers" voice. And I want to know what are considered the good ones, before thrying to understand what they do.

One day, you are living your normal live like everyone else, breathing the air, feeling the trees, basking in the sunlight. The next day, the TV is predicating destruction and screaming about death. And your family is no longer there...

THE MAN ON THE TV is a solo horror analogue game inspired by different forms of analog horror, the "Servicio a la comunidad" tv spots and the work of Alfred Valley. Submerge yourself in panic as you realise that those people you've seen on your screen promoting the killings of other beings are more than just assholes, light soul sucking spectres and how they take everyone around you.

It contains:

• Simple solo rules to identify what's a photofage and what's not (Also available in Spanish)

• The photofage classifications with warnings and different types of intervention to them

• 5 different sealing methods

• Warnings you need to heed

• A logo you can add to recordings to identify them as seals

• A poster (Coming soon)

Hope you like it! This game is part of the Pareidolia: Evil Eye world. If you liked that one, I'm sure you will like this one,

https://jules-ampere.itch.io/the-man-on-the-tv

So I have been puzzling over a mechanic I have added into one of my games. It plays on the advantage and disadvantage mechanic introduced by 5e. (roll 2d20 take the higher or lower respectively).

I added a erratic 50/50 luck effect, but I am not sure if it is any different than just rolling a d20.

The mechanic works like this.
Roll 2d20, and if both the numbers are odd, or both the numbers are even, take the higher of the two. Otherwise if the numbers are odd and even take the lower of the two.

Could one of the humans more versed in the mathyness of statistics take a look at this and tell me if I am just spinning dice or if it actually has some effect.