The Formosan chest is a plot device in Return of the Obra Dinn. It is a cabinet that has two compartments: The top part has a pedestal attached to a lid that covers a compartment with liquid substance. The bottom part has a drawer that can fit a shell. The origins and the purpose of the chest are also not explicitly revealed in the narrative.
The exact nature of the substance the chest contains is not explained. In Bargain, part 1, Fillip Dahl says the contents look like quicksilver (metal mercury). However, unlike mercury, the substance burns or corrodes human flesh on contact.
The chest is one of the items the four Formosan passengers are taking with them aboard the Obra Dinn in 1802. The chest was initially held in the passenger cargo hold, guarded by Hok-Seng Lau. Second Mate Edward Nichols wanted to gain access to the chest, and knocked Lau unconscious in Murder, part 1. He later led a group of mutineers that stole the chest, took the Formosan royalty captive, and set out to the Canary Islands in lifeboats.
The only commentary regarding the nature of the chest (besides a brief remark by Dahl in Bargain, part 1) is given in The Calling, part 1 where passenger Bun-Lan Lim tells her fellow Formosan It-Beng Sia that the chest must not fall into the ocean. Later, the mutineers are attacked by mermaids. During the conflict, Sia takes a shell from the bottom compartment and places it into the liquid in the top compartment. This releases three beams of energy that hit the shells the attacking mermaids have, stunning the beasts. It is not explained whether the mermaids are after the chest or the shell it contains. It is also not explained what the beams that emanate from the chest are.
After the battle, the only survivor, Edward Nichols, takes the lifeboats, the mermaids and the chest back to the Obra Dinn. Nichols, however, is shot on sight by the remaining Formosan passenger Chioh Tan. With the treasure back on board, seamen Alexander Booth, Lars Linde, and George Shirley carry the chest to the lazarette. The chest is not seen again until chapter VIII, Bargain, after the player finishes the 58 possible fates.
Although not directly shown, the chest presumably sinks with the damaged Obra Dinn shortly after the inspector has left the vessel.
Unlike the corpses of Filip Dahl and Martin Perrot, the 1807 version of the Obra Dinn does contain the chest within the lazarette, where it was left in Bargain. The unknown liquid contents of the chest's top compartment are not shown, but as the lazarette is not intended to be accessed in this manner and the chest is not normally visible, this may simply be to avoid needlessly rendering a complicated visual effect.
- After placing a shell into the liquid, it burns passenger It-Beng Sia's arm off to the bone in The Calling, part 5.
- When taking a shell out of the liquid, captain's steward Fillip Dahl burns his arm in Bargain, part 1.
- In Loose Cargo, part 1 although the room cannot be accessed at that point.
- In part IV, The Calling
- In The Calling, part 4
- In The Calling, part 6
- In Unholy Captives, part 4
- The final insurance assessment book mentions that the ship sank in Falmouth and all cargo was lost.