Some miserable small time necromancer was seen graverobbing and fled into the catacombs beneath the cemetery in the middle of a downpour. Just as the party pursuing him reached the lowest depths, the levies broke. And now the floodwaters in the tombs are rising, and they're rising FAST. Find a way out while you can, in waters that are thick with the undead, before you join their ranks.
Work your way to the "boss" fight, and then force your way out through rushing waters filled with hungry ghouls. Play the escape with a timer going to keep the pace moving maybe.