Mannimarco in Oblivion. Or at least have it retconned to state he was merely the mortal aspect of Mannimarco left over after the Warp in the West, and that the God of Worms still lives on.
That quest made me stop playing for over a year. Sickening.
Though it doesn't need to be retconned, just clarified; Mannimarco in OB was a random necromancer. He was not a Lich (The Warp in the West would have had no effect on the KoW's lichdom). Without the God of Worms, you can't have black soulgems, so that part of the Warp must have occurred.
The other interesting bit is when you consider the canon at the end of Oblivion. Is it assumed all guild storylines occurred? Obviously the CoC could not have been the leader of all guilds simultaneously; it stands to reason that either the quests were completed by unknown individuals, or they are not canon at all.