Something deep within the dungeon takes note of how the adventurer who has been easily killing all its monsters has turned away from the boss room, signaling her intent to stay upon its first floor. Under other circumstances she might have been given more of a grace period, but in this case, a certain additional factor is in play as well.
Some time after Bella and Haroun have left behind the antechamber to the boss room, the might tin-bound doors swing open, and a small pack of monsters move out with surprising speed and stealth.