Revenats are a particular and peculiar form of Undead. They are generally created when someone dies with a great task left unfinished. Unlike many undead, they are generally not evil, and a few have been known to go on to become great heroes.
One particular peculiarity of revenants is that they don’t always return in their own bodies. Some inhabit the bodies of other dead people, or even create whole new forms out of corpses that bear some rememblance to who they were in life, but this may not be an exact match.
Their undead existence bestows a number of powers upon them, and these are sometimes geared specifically towards the quest that they had left undone.