In our book Character Foundry, we discuss the concept of a permanent enemy. Permanent enemies are those who keep coming back to haunt the players/party again and again. We had several good ideas on how to keep bringing them back, but here are a couple more:
What about resuscitating them? So often player characters get raised from the dead, why wouldn’t their enemies? For this to happen, someone would need to be able to reach or retrieve the body(ies), and someone would need to cast the spell(s) capable of bringing them back. If the PCs can do this, why wouldn’t the NPCs? As long as the PCs can do this as well, it should be possible for the bad guys to do it as well. This can be dangerous, because once the party figures out that the bad guy can be brought back, they are going to start doing nasty things to the corpses before they leave the area. You may need to hide this guy in a helmet, at least at first. (Game rules point - At least in Legend Quest, you cannot return life to a body incapable of maintaining it. Thus, you cannot bring back a body that has been quartered or decapitated, nor could you help a corpse that had been burned, even if it hadn’t been completely consumed.)
OK - So you cannot keep bringing them back from the dead - the players won’t go for that. So the next one is similar - Instead of bringing them back to life - only take them half way - go for the undead. Bringing them back as a zombie is probably useless as we all know zombies are far too weak. But if you bring them back as zombies and try to hide the fact (making people including the PCs believe them to be back from the dead), then it can be fun. Still, that might just piss off the players when they realize they were duped. Depending on your game - Turn them into an undead or a lich if you must use that word. Whatever it is called, turning them into some manner of undead with intelligence probably brings them back into the game while giving them extra powers they did not have before, thus making them ready to face the PCs again. If you could turn them into a vampire, they could turn into mist clouds etc. and stay on as permanent enemies. Otherwise, being undead gives them a second chance but probably not too many more. You can probably bypass some of the rules about creating these kinds of undead if their god or demon wants them back in the action badly enough.
Of course you could just keep locking them up in an asylum that they all keep breaking out of, but who would be stupid enough to get involved in a story with that plot point. Oops