Adjudicator of Sin

[Egon the Monkey]

Suggestion: Change their defensive spell from Improved Invisibility to Displacement.
These are a pretty cool monster, but after they fire off their dangerous spells, they tend to sit plinking at you with Acid Splash for 10 minutes while you fail to hit them and they regenerate past damage. If they had Displacement instead, they would still be defended while casting, but it would wear off once they start running out of spells.

They'd still be just as tough, but you wouldn't spend the last bit of the planar invasion quest grumbling WHY WON'T YOU DIE while taking negligible damage from a  worn-out  caster XD