After figuring out a way to disarm the trap on the iron-bound door, the explorers discover a room with an open pit of lava at the center, and several suspended vats hanging above three of them feeding into a fourth at the center, just about the lava.
Each of the three vats has a valve held closed by zombie-arm, such that if the magic fails, the arms would release and let the components mix before dropping them into the fiery rock to explode. With a bit of work, Qing gets Olivia under the saltpeter vat, while Morgan, Natasha and Qing's faux-Valicross zombie empty salt from the zombie's coffin so it can be used to drain the powder into.
With things in place, Olivia digs out a hole in the vat with her knife, and it begins to drain. The nymph finally reappears with the crystal it was commanded to get, and this time it accuses the fake Valicross of being a fake, and tries to get the crystal into the bomb room.
The zombie-spirit tackles the nymph, and Morgan recovers the crystal before it could end up in the fire. But then the nymph triggers the bomb-machine, and everyone runs even without the saltpeter, there's still enough powdered charcoal to create a good boom, which knocks the lab door off of its hinges.