A Death Knight posessed by a Noble weapon
The Sickle is a Death Knight, his flesh all but rotted away before the undeath took hold, with an almost-permanent rictus-like grin on his face, with two burning red pinpoints for eyes. He stands almost 8 feet tall and rides a Clydesdale into battle, his white robes with gold trim flowing about him.
Grimskull was an ordinary Death Knight, having been cursed in unlife to fight forever, waging a pointless war across the Shadowfell until he took as the spoils of war from one of his conquests the Golden Sickle. This intelligent, holy weapon impressed its will on Grimskull, advising him to take his soldiers, mark them as his own, and wage a war against the darkness that pervaded the parallel plane. His Shadar-Kai troops were only too happy to oblige, the sickle’s light sustaining them in the dark land.