You can choose to move to the endgame at the start of any round once at least one stack has been emptied of all cards. However, you are not forced to start the endgame unless all three stacks are empty. Set up the endgame as follows:

ACTIVATE A BOSS. If a boss is already revealed (some rewards and effects cause this), activate the revealed boss. If not, activate one of the four bosses at random. Read the boss’s card in full.

ACTIVATE THE ENTOURAGE. Every boss card has on it. Activate the indicated number of enemies from the stacks as normal (mandatory enemies must be activated over non-mandatory enemies). Remember to activate any enemies added as a result of incomplete missions as well. Ignore on entourage cards. If there are not enough cards remaining in the stacks to fill the entourage, take all of the conflict cards in the discard and shuffle them together into one new stack.

The endgame is played according to standard endgame rules. When the boss is defeated, the game is won, even if the boss’s entourage remains. Some bosses may have additional or alternate win conditions.