The Defiler is a Fallen in Oninaki. It is a boss and is fought on level 75 of the Sanctum of Rebirth.
Log entry[]
Horrific Fallen comprised of myriad masked countenances that unleash a unified onslaught once each face is ready to strike. Their irregular formations make it hard to fall into a rhythm, but they don't behave much differently than the base species, so adapt to the battle as it unfolds.
Page 2
Oh dear. Dear, dear, dear. Did you hear the news? Do you think it's true? How terrifying. I never expected something like that. Who knew... Speaking of the unexpected, the other day - my, yes, what a tale. But it's nothing to do with any of us.