What I saw one time was some other king of rejuvinating undead that was pretending to be the lich.
The party was going around smashing everything trying to destroy the phylactery. Only their wasn't one.
I can't remember what the method to kill this thing was, might have just been killing blow had to be positive energy.
Hmm Demon Knight and Death Knight really aren't the same are they. Cauchemar-mounted DK, plane-hopping his lich master around just gives me the shivers.You'd probably want to use a graveknight. They're pretty much the deathknight equivalent for Pathfinder. They're bound to their armor instead of their weapon though. The insidious thing about a graveknight is that is can regenerate in the armor while someone is wearing the armor, and the person's body eventually becomes part of the graveknight (according to Undead Revisited).
Not sure what a demon knight is. Guessing you meant Hellknight.
Ah I meant Demonic Knight. Haven't been playing PF long so it was a rushed search to find the death knight.
Looking over Grave Knight - yes that would do nicely! :)