During the Obra Dinn's last voyage in 1802, the good ship was assaulted by various hostile creatures of the deep that the book generalizes as "terrible beasts". They were all most likely drawn in by the presence of the shells.
Mermaids[]
Lured by the presence of the chest, Second Mate Edward Nichols, his fellow mutineers, and the kidnapped Formosans were assailed by three mermaid-like creatures. They possessed dark and messy hair, sharp claws on their left hands, a banded tail, and spikes on their backs, which they can launch out as deadly projectiles. They appeared to have three pairs of visible breasts. Some wielded spears.
When the mermaids attacked the boats, one of the kidnapped Formosans, It-Beng Sia, opened the chest and activated its mechanism, which summoned columns of unknown energy that stunned the mermaids. Nichols then hauled them onto the lifeboats. When the lifeboats were reclaimed by the Obra Dinn, the mermaids were hauled up in a net. However, by this point their stunned state was beginning to wear off, and one mermaid indiscriminately shot spikes at various crew members. The mermaids were re-subdued and placed in nets to be carried to the lazarette, as instructed by Captain Robert Witterel. On the way to the lazarette, one mermaid fatally struck the cook with her tail, causing the seamen carrying her to lose their footing and fall down some stairs. William Wasim, one of the seamen carrying the mermaid, snapped his neck. Fillip Dahl, convinced that the mermaids carried a curse, attacked seaman John Naples, perhaps in an attempt to free the mermaids or dissuade the crew from taking them.
Following the mermaids' captivity, several other beasts assaulted the Obra Dinn, also drawn by the shells. While the Kraken attacked the ship, Captain Witterel held the mermaids responsible for summoning the Kraken as a rescue call. He ordered them to call it off, killing two mermaids in the process. Captain Witterel is also implied to have thrown two of their shells overboard.
After the last remaining mermaid called off the Kraken, Third Mate Martin Perrott was joined by stewards Paul Moss and Davey James to set this surviving mermaid free. Immediately after opening the mermaids' cage, Perrott was spiked before he could convey his intentions to the mermaid. He ordered the stewards to give her the last shell and throw her overboard. In return, the dying Perrott asked the mermaid to see the Obra Dinn to safety. The mermaid fulfilled this final promise by somehow bringing the Obra Dinn back into port, though years later and without any live passengers aboard.
The shine/aura of the shell given to the surviving mermaid can be seen portside in the distant sea by the investigator when they are not using the pocket watch, suggesting that she was watching from afar during the entire investigation.
Crab-riders and giant crabs[]
Shortly after the mermaids' captivity, two crab-riders boarded the ship and fought their way towards the lazarette.
The crab-riders were humanoid in figure, but were covered in a hair/seaweed-like substance that obscured their faces. A pair of glowing eyes were visible from underneath their exterior. They primarily wielded spears, but could also apparently draw the spikes on the back of their mounts to throw like daggers with enough force to impale a human being into the ship's wooden sidings.
The giant crabs looked similar to spider crabs. They had spikes protruding from the back of their carapaces that could be ejected to kill prey. They also used their claws to attack.
Unlike the other two beasts, the crab-riders are seemingly not based on any existing or established nautical lore.
Kraken[]
The last and largest beast to accost the Obra Dinn was the Kraken. After the crewmen killed the crab-riders, the mermaids called on the Kraken to attack.
Only the Kraken's tentacles were visible. Its tentacles seemed to be surprisingly dexterous, as it managed to locate and interact with various crewmen and objects. It retreated after Captain Witterel killed two of the mermaids and threw two shells overboard.
Aside from that of Lars Linde, the Kraken was responsible for the deaths of those who perished in The Doom, and it has the highest body count of all the beasts that attacked the Obra Dinn.
The Kraken was also the only beast that was mentioned by name.
The overall shape and appearance of the Kraken is unknown, only its large tentacles, similar to those of an octopus or squid, are shown in-game. In The Escape, Part 1, Alfred Klestil refers to it as "the squid", but it is likely that he is guessing based on its tentacles. In real-world mythology, the Kraken is typically depicted as a large squid or octopus.
An image of the Kraken from The Doom Part 8, taken using debug and NoClip. Only tentacles are depicted, the Kraken's overall body shape remains unknown, though is suggested to be a squid in The Escape Part 1.