The leader of this dungeon comes back to life when killed "as if he was a newly spawned monster." I assume this means that he has to appear outside the line of sight of all the heroes, but what happens if the heroes have the entire dungeon covered? I guess that's one way to kill him permanently?

Its in the FAQ, but not with the rest of the Dungeon level fixes. Sloppy editors I guess.

Q: Ancient Library (Dungeon 10): Does the leader reappear in his original spot or respawn as per normal spawn rules?
A: He reappears in his space, pushing aside any model in that space. The Overlord chooses where the pushed model is placed, up to 1 space away. No, this can not trigger movement traps.