You see a sin devourer
Notes
Apppears in the Ferumbras' Ascension Quest with Shulgrax.
Abilities
Mana Drain (100-200)
Debuff (Reduces Magic Level
by 40-50%)Damage Taken From Elements
Physical 
Death 
Holy
Ice
Fire
Energy
Earth
Location
Grounds of Damnation in Shulgrax's room.
Behaviour
Keeps a distance.
Strategy
They can't be killed directly, so to get rid of them you have to kill the Damned Souls first and then use the Purified Souls on the Sin Devourers.