If the heroes destroy the Catacomb Lord, and at least one hero remains alive, the heroes win the game. Heroes only need to destroy the Catacomb Lord to win. They do not need to destroy all monsters in the Catacomb Lord’s Lair.
If at any time during any Battle Phase, all four heroes are killed and/or are unable to take an action, then the Overseer wins and the game is over. Heroes may be unable to take an action, for example, if they are trapped by a Devoured card.