Cursed Undead

A cursed undead, soul bound to his corpse even after he's dead.  For a while they might even know they're dead for years, but the symptoms become obvious.  A lack of hunger, inability to sleep...  but they're dead that can not pass on.  Slowly rotting from the outside, trapped inside, able to feel it.  They eventually go mad.  While not all turn to necromancy in attempt to find a solution, many do.  Being dead makes them naturally attuned to necromancy.  They should not be underestimated.  Your best bet, should you encounter one of these creatures, is to disassemble it, and not allow it to crawl back together... until a cure can eventually be found