The PUG ran into one of these little buggers this evening, when they went around opening spell sealed sarcophagi in a cursed tomb. They were first seen in the 3.5 Monster Manual IV , and to be honest, hadn't really registered with me at all until I was flicking through that tome a few weeks ago, looking for things to throw at my players. It's actually a pretty damn creepy monster when you think about it, especially the part where is forces you to become one of its screaming victims for a short time.
It nearly killed the group by the way, the Frightful Keening keeping most of the group at a safe distance, allowing it to flit here and there, taking down the party one character at the time. Luckily they just managed to wound it enough that it fled....just.
Defacer – Medium Undead
A.C. 15 (Natural Armour)
Hpts: 120 (16d8+48) [64 - 176]
Speed: 30' (Earth Glide)
Proficiency Bonus: +3
Str: 18 (+4) Dex: 15 (+2) Con: 16 (+3) Int: 7 (-2) Wis: 13 (+1) Cha: 18 (+4)
Saving Throws: Constitution +6, Charisma +5
Skills: Deception +7, Perception +4, Stealth +5
Damage Resistances: Psychic; Slashing, Piercing and Bludgeoning damage from non-silvered weapons
Damage Immunity: Poison
Condition Immunity: Exhausted, Poisoned, Unconscious
Senses: Darkvision 60', Tremorsense 30'; Passive Perception: 14
Language: Language spoken in life
CR: 7 (XP 2,900)
Earthglide: The Defacer may move through solid rock as if it did not exist, leaving no tunnel or trace of its passing. It also ignores any difficult terrain that is the result of rocks, rubble or similar earthen objects.
Frightful Keening: The faces that surround the Defacer constantly emit a horrific, hopeless wailing, which carries through solid stone as if it were air. The Defacer cannot stop this sound, and any creature that comes within 60' of it is Frightened. However, they may still approach the Defacer if they make a D.C. 15 Wisdom saving throw (though they must do this each time they try to close the distance between them and it).
Multiattack: The Defacer makes two Distorted Strikes
Distorted Strike: Melee Weapon Attack, Reach 5', One Target, +7 to hit, Hit: 13 (2d8+4) Bludgeoning +9 (2d8) Psychic damage and target must make a D.C. 15 Charisma saving throw or be Stunned until the end of the Defacer's next turn. Whilst stunned, their visage warps into that of one of the souls captured by the monster, which begins to scream for help in the soul's native tongue.
Steal Face: If the Defacer kills a creature with an Intelligence of 3 or more, or if it touches the body of such a creature within 1 day of its death, it may steal that creature's soul as a bonus action. Once this has happened, the Defacer's own face takes on their appearance for 1 day, and it is calm and sedate for that time. If it is forced to use its Earthglide power in that time however, it looses the face (which then becomes another screaming soul surrounding it). If the Defacer is slain whilst it wears another's face, that individual returns to life, stable, at 0 hit points.
Souls that are captured by the Defacer are not free to return to their bodies, and are not free to answer spells such as Speak with Dead.
I've heard about folks being two faced, but this is taking the piss...