The majority of cursed undead NPC or PCs are the result of someone hating or desiring revenge on the character so much that they search out a witch or necromancer to curse them. The curse keeps the character trapped in their soulless dead body, imprisoned forever in an unfeeling and unliving facade of life, or at least until their body wears out or is destroyed.
Cursed undead retain a good portion of their memories, but their feelings and morals are usually dulled by their undead existence. Some cursed undead maintain a facade of life, while others allow their body to deteriorate until they resemble the ghouls and zombies they share undeath with.
Use these stats as the base for Cursed Undead