From the introduction:
Starting when the adventurers enter through the iron doors, the Dungeon Heart initiates its Plan, summoning Minions as listed below, multiplied by the number of adventurers. Summoned Minions move towards the Dungeon Heart and any adventurers inside along an invisible path (purple) in single-file and unaffected by obstructions. If their attack wouldn’t reach an adventurer, they move a 2nd time instead. They ignore any ghostly Heroes. Minions on the map attack adventurers they see, pursuing until victory or death.
The adventurers will need help to not be overrun, and spirits of long-lost Heroes fill this place. Luckily, those Heroes can be empowered to help the adventurers fight off the endless Minions closing in behind them.