[Endoperez]

They don't. Except if they happen to be skilled in Nature and/or Death, or do some of the rituals that help against aging, or wear an item that helps against aging.

That's a bit like saying that there should be a way to kill off a Vanheim Dwarf that's cast Invulnerability on himself, because he might survive being swarmed by a bunch of Principii for the 45 turns it would take for the forces of Pythium to retreat. Yes, it could happen, in theory, if nothing else happened (like the Dwarf routing, or becoming unconscious, or enemy mages casting something, or the Dwarf casting something), but it won't. And if it will, it will be the exception that makes this game wonderful, or frustrating but interesting. Like that time I lost the turn I got two Great Treasures, giving me Robe of the Magi and the last item I lacked from the equipment of the Archdevil (regeneration) I was 1 Blood booster away from summoning.