Well, my idea might be hard to implement.
A warlock is just a human living creature, up to a certain point. When they've mastered a certain spell or built a certain workshop, they can progress to becoming an undead lich.
When they're a normal warlock, they are subject to all the problems that any living creature has. Losing body parts, dying, eating and sleeping, etc.
But if they master these certain magical arts, they can transform into an undead lich which while not invulnerable, is impossible to permanently kill. So long as there's any controllable minion in the fort at all, they can bring the undead lich back to incorporate a new form.
It fits that "vanquished but not defeated" meme you see all the time in movies and literature.
ETA:
One way I tweaked this was to edit the raws so that the overlord caste (which never shows up on its own) is transformed into one of the undead. He gets the NODRINK, NOEAT, NOSLEEP, etc. tags and I increased his physical toughness a little bit.
My understanding of the theme of the warlocks is that everyone in the dungeon is there to serve the overlord. The loss of a minion, even an important minion like a necromancer, is not that big of a deal in the long run. The loss of the overlord is the loss of the fort though, and I don't like my epic fort dying because a kobold thief lopped off my overlord's head with a large copper dagger while he was standing by the bloodwine stockpile.