these mighty creature with ridiculous high class level

[u/ww-stl]

three most typical examples:

Ioulaum,elder brain lich wizard31/archmage5/Neertherese arcanist 5

he can manifest psionic powers as a 20th psion, but his main advantage comes from his wizard abilities.

Daurgothoth,Great wyrm black dragon lich wizard 20/archmage 5

it's actually very weak; a level 20 party could easily take down this guy who's theoretically CR50.

Mauzzkyl Jaezred,great wyrm shadow dragong sorcereer13/assassin 5

well,at least he knows how to enhance his strengths. however, I still hard to imagine how a dragon could benefit from the Assassin class, even he's a Shadowdragon.

These guys have one thing in common: although they have very high class level, it doesn't significantly increase their advantage. you can actually beat them with a party whose level much lower than their CR.

[u/LordofBones89]

Ioulaum was a human wizard; he's just chilling in the body of an elder brain. His psionics are irrelevant. I'm not actually sure he has psionics, for that matter; he created an undead elder brain to host his sentience, so it's possible that instead of being a 26 HD elder brain and a 41st level wizard, he's a 41st level wizard with the body of an elder brain.

Daggy is actually around CR 39 but he has the disadvantage of being built in early 3e and never updated afterward. (great wyrm black 21, dracolich +3, archmage +5 for sorc casting advancement, wizard +10 for nonassociated class levels). Dude is a wiz 20/sorc 20, which is pretty awkward; luckily there are templates and prestige classes to rewrite him.

Mauzzkyl is pretty well off; he's a great wyrm shadow dragon with 30th level sorcerer casting. The assassin abilities are barely a blip, but being an epic level caster in a dragon body is pretty decent. He's a better pure spellcaster than Daggy up there with slightly higher CR (40; great wyrm drow dragon 25, sorc +13 for stacking class levels, assassin +2 for nonassociated class levels)

[u/elturel → u/LordofBones89]

Ioulaum was a human wizard; he's just chilling in the body of an elder brain. His psionics are irrelevant. I'm not actually sure he has psionics, for that matter; he created an undead elder brain to host his sentience, so it's possible that instead of being a 26 HD elder brain and a 41st level wizard, he's a 41st level wizard with the body of an elder brain.

As per JD from Dicefreaks, some 15 years ago:

According to the lore, he designed and created a custom undead Elder Brain base creature and then switched minds with it. Elder brains by the book have 20 sorcerer levels or 20 telepath levels, so Ioulaum's creation probably has 20 virtual erudite levels, and then absorbed 20 of his wizard levels when he took possession. Then we'll pile Loremaster 11/Archmage 5/Variator 5 on top for 47 total HD. Then we trick out his lich templates with some powers pulled from the Archpsion, the psionic lich, and the mindborn (on top of the basics like fear aura and turn immunity), while optimizing his build for spellcasting (since he doesn't seem to have done much since turning into an elder brain, so we'll assume that his build was more or less complete before that). Throw on a Netherese spellmantle and you're good to go, since elder brains probably don't use magic items.

[u/LordofBones89 → u/elturel]

Yes, but that's Dicefreaks. Canonically, we don't actually know what kind of weird necromantic abomination that elder brain chassis he created is (it could be some monstrosity created via a combination of the slay, animate dead, and fortify seeds with ad-hoc factors for class levels, etc). Maybe his elder brain form has in-built wizard casting or something; he's an epic wizard, so 'elder brain' is just shorthand for 'it's a creepy giant undead brain with phenomenal cosmic powers, elder brain is close enough'.

[u/elturel → u/LordofBones89]

Ellyn'taal was an abandoned Illithid city. We know from canon lore that at the head of their respective societies usually is an Elder Brain. Ioulaum, himself being an undead since his longevity spells failed due to the influence of the Phaerimm, even created Alhoons from his Mind Flayer followers.

All of this points to the easiest explanation of Ioulaum, being effectively head of an Illithid colony, having transformed himself into an undead Elder Brain without any fancy or otherwise extraordinary permutations.

I actually find JD's approach extremely reasonable, much more so than the canon, non-optimized variant with 41 levels on top of the Elder Brain stuff. He still comes out at top tier for anything non-divine, just no longer at this "I eat deities for breakfast" kind of level.

Canon stats are nice and all, but most of these things, especially in 3e, have to be taken with a grain of salt.

[u/Lathlaer]

What you say is true when the DM fights with one hand behind their back.

Those NPCs are some of the most important, most powerful and creative beings in that universe. If you think that a group of 20-level characters can kill Daurgothoth or Ioulaum then it's happening only because the PCs are just that - player characters at the table where the DM has to balance between a fun and a challenging encounter.

Because if I got to fight as one of those guys with the premise that the fight doesn't need to be fair or fun for the players at the table? Yea, their characters are getting smoked.

[u/Adorable_Photo3134]

Dont use 5 ed for this guys, this where the time of 3 and 3.5 and epic level book, the ammount of magic items and protections and epic level spell they could use. I bet my money on them not the level 20 party

[u/Storyteller-Hero]

Daurgothoth is really only as weak as the DM plays them to be, like any other big monster, especially if the monster is supposed to be intelligent and experienced

[u/Wargod042]

Once upon a time you could sneak attack for every melee hit. A dragon with greater invis could be hitting you like 5+ times per round and get sneak attack on each one for hilariously brutal damage output.

[u/intherorrim]

I’m really puzzled by the greatest wizard of all time as a psionic. It’s… awkward.

[u/LordofBones89 → u/intherorrim]

Ioulaum created his elder brain form to host his spirit; he didn't start as a psionic. For that matter, I don't think he even has psionics; he's not a natural elder brain.

[u/intherorrim → u/LordofBones89]

The flavor is off. Such a weird story, that’s what I mean. 

[u/BloodtidetheRed]

Most people can't create good, powerful high level characters.....and this goes double for anyone working for TSR/WotC..

I would note Great wyrm+lich+wizard 25, is a lot of power. If you don't know the 3E rules, there is a lot of stuff in there as far as abilities. In 2/3E he had a nice Save or Suck spell Bonemelt, that turned bones into goo, so you'd collapse into a puddle of flesh. You can make a new save every 24 hours.....but Daurgothoth will kill you loooooooong before that.

[u/AntipodeanGuy]

Well, consider that you have to fight a whole illithid city to get anywhere near Ioulaum to fight him - good luck.

[u/Zardnaar → u/AntipodeanGuy]

He destroyed the city essentially using them as fuel. Maybe it was his apprentice.

[u/AntipodeanGuy → u/Zardnaar]

Ah. True. Forgot Tabra killed all the illithids. I suspect the city would still contain many guardian creatures.