Revenants

A revenant is the vengeful spirit of the fallen, usually seeking to enact revenge or handle unfinished business.

Vengeful by Design

Revenants seek revenge for the wrongs they have suffered, and their pursuit of revenge is wholly unwavering. Once created, a revenant will continue to pursue its quarry at any cost. A revenant whose body is destroyed will seek out another corpse to animate, gaining the abilities and knowledge wielded by that corpse in life.
Revenants are unparalleled hunters of their prey and have a persistence magical sense of their target's location.

Variants

Dullahans

Dullahans, commonly called Headless Horsemen, are the wrathful spirits of decapitated creatures. They wield cursed weapons made of bone, and have an unusual weakness to gold.
Legends claim a dullahan can be harmed only by gilded weapons, and that dullahans cannot be destroyed until a gold coin is placed in its mouth.

Druagar

A powerful quartet of spirits used by orcish clans to enact vengeance on their foes.