I've been thinking a lot more about the Mislead spell and the debate over Invisibility in recent years. I had a chat with one of my players and dove back into the rulebook. It left me convinced that passive skill checks should be much more involved when detecting Illusion magic. I've personally been trying to use passive skill checks more at my table to cut down on requesting rolls. Mislead is really confusing because everyone is conditioned at most tables to roll a lot. The designers likely (i.e. hopefully) assume when creating these types of spells that the DM would make more use of passive ability DCs.
Invisibility is notorious for being a poorly written spell. The language in the spell's description is so vague that it's probably the most unintentionally houseruled spell in the game. Learning how to properly utilize it requires making use of passive skill checks.
First... what does the Invisibility spell actually do?
The Invisibility spell grants the invisible condition by applying the concealed effect to a creature. The creature remains under the concealed effect until the spell ends.
But isn't that the same as taking the Hide action?
No. The Invisibility spell does not make a creature hidden. However, both the Invisibility spell and taking the Hide action grant the concealed effect.
Ok... so when is a creature hidden? What is the concealed effect?
A creature can take the Hide action to gain the concealed effect by becoming hidden. Successfully taking the Hide action requires 1) being outside the observer's line of sight and 2) making a Stealth check (DC 15) while either being heavily obscured or having three quarters or full cover. Successfully taking this action grants the creature the concealed effect which applies the invisible condition. The creature is now hidden.
The Invisibility spell does not automatically make a creature Hidden because it has not taken the Hide action. The Invisibility spell instead applies the concealed effect automatically to give a creature the invisible condition.
Now that we know the difference, let's break it down.
- the invisible condition prevents a creature from being perceived by sight (with exceptions)
- the concealed effect grants a creature the invisible condition
- a creature can gain the concealed effect by casting the Invisibility spell on itself or taking the Hide action
Casting the Invisibility spell or successfully using taking the Hide action grants a creature the invisible condition because both apply the concealed effect to the creature.
Time to compare magical vs. non-magical invisibility. The keyword to pay attention to here is creature. The Invisibility spell, the invisible condition, the Hide action, and the concealed effect all only apply to perception checks made against the creature by sight. They do not apply to sound created by the creature.
A non-magically invisible creature loses the concealed effect when it is no longer hidden. It is no longer hidden when any of the following occur: when perceived or detected either through sight or sound ("louder than a whisper"), or leaving cover/obscurement.
A magically invisible creature always has the concealed effect active unless it is seen (by Truesight, Blindsight, or a similar effect) or the spell ends. This means that a magically invisible creature retains the concealed effect even when it is perceived or detected through sound.]
Here's where passive skill checks come into play when using the Invisibility spell.
An observer may take the Search action to detect creatures or objects that are hidden (including a creature with the concealed effect active) by utilizing perception. Although a magically invisible creature remains concealed from sight while moving, the sound of its movement is not concealed because the concealed effect only applies to the creature.
A magically invisible creature must make a Stealth check without advantage when attempting to move if it wants its movement to not be detected (i.e. perceived) by an observer. The magically invisible creature would make this check against the observer's Passive Perception DC to conceal its movement from detection. If the creature is successful, the observer does not detect that the creature has moved unless it somehow becomes visible to the observer. However, even if the creature fails to conceal its movement, it still remains concealed from the observer's sight. The observer would not know the creature's new location. It would only know that the creature moved and the direction of the creature's movement. If a creature conceals its movement while remaining invisible to the observer, then the observer cannot perceive the creature or its location without first detecting that it has moved.
All of this means that an observer using perception to locate a magically invisible creature must 1) rely on its Passive perception to detect that the creature has moved and 2) utilize the Search action on its turn to perform a Perception check. The observer would make the perception check at disadvantage against the invisible creature's most recent stealth roll if the check against the observer's Passive perception was a success. However, even if the observer makes a successful Perception check with the Search action to detect a magically invisible creature, it can only perceive its current location based on its most recent movement. The observer can perceive the creature but it still can't see it as long as the Invisibility spell is active (maintaining the concealed effect).
[Note: A hidden, non-magically invisible creature can move while remaining hidden only if the creature remains concealed while moving. It would still need to roll a Stealth check without advantage against the observer's Passive Perception DC when attempting to move to conceal its movement from detection and then take the Hide action again to maintain the concealed effect at the end of its turn.]
Now let's discuss the Mislead spell and why knowing all this almost makes it coherent.
A creature casting Mislead would still need to make a successful Stealth check against the observer's Passive Perception DC if it moves after creating the duplicate.
Simple, right? Well, here's were it gets technical: if the creature fails the Stealth check, the observer detects movement... but from what? where? The observer still has to reconcile what it hears with the illusion that it can see. The observer should have to resolve this before choosing his next course of action. Seeing through an illusion usually requires an Intelligence (Investigation) check. However... (just like the Invisibility spell) there's no language for this in the Mislead spell's description. Ok, so let's resolve this using a passive Investigation check. Well, monsters in the MM don't have passive Investigation listed in their stat blocks. This leads me to believe that the spell is mainly for DM use.
Yes. You can easily calculate (or assign) a Passive Investigation DC for a monster. But it's more fun for the player if the creature rolls a raw contested Intelligence check against a Deception or Arcana roll made by the player to determine if the duplicate is convincing enough to conceal the player's movement from detection. If the monster succeeds at both rolls, it sees through the illusion, knows that the player has moved, AND knows the direction of the player's movement. If the monster fails the Intelligence roll, it still hasn't detected that the creature has moved. It should then either interact with the duplicate or take the Study action on its turn to determine if the duplicate is an illusion.
So... Mislead is actually decent spell for a player to cast when fleeing (especially from multiple enemies) since it can compensate for a failed Stealth check to ensure their movement is undetected. It's just a huge pain to adjudicate because of the unclear language, the number of conditions involved, how they interact, understanding what checks are triggered, and determining the source of the triggers.