Curse.
It's spoken of as something inherently fiendish in nature.
However, the only enemies that inflict it are undead.
Shouldn't be too much of a stretch to introduce curse themed greavers and Trojans, for starters. It could fit thematically, and the curse could be pared down to a 2 vial 1 weapon one or the monsters can be extremely rare.
But, yeah, something that annoys me. Maybe Legion of Almire will have some Curse dealing fiends.
"curse themed greavers"
*shudders*