A new “illusion” or “disguise self” monster spell, which makes the caster or an allied monster appear as another monster of its choice until something disrupts the illusion (it gets attacked, otherwise takes damage, or it makes an attack). Monsters specifically cast it when you cannot see the target monster. It might be possible to implement this spell to be player-castable as well, but this would be less effective. (you could use it to fool monsters into thinking you’re another hostile monster so they don’t attack you, but not much else).

The illusion favors nasty monsters, but low-level spellcasters may only be able to create illusions of somewhat higher-difficulty monsters. A potentially evil addendum is that high-level spellcasters may use the spell to make themselves or other powerful monsters appear to be weaker monsters. Any bosses that use this spell may use it to make themselves appear as a weak monster so they can take you unawares.

Perhaps if you have extrinsic see invisible (and only extrinsic), you can see through such illusions.