Sea Emperor Leviathan

"What.... are... you?"

- The Sea Emperor to the player via telepathy. The Sea Emperor Leviathan, also referred to as the Sea Emperor for short, is a supporting deuteragonist in the video game Subnautica. While it was spawnable in the Early Access version around 2015, it never got a proper role in  the story until late in 2017, her story being fully accessable in the full version of 2018.

About the Species
The Sea Emperor Leviathan is one of the many Leviathan-class organisms on Planet 4546B, the planet Subnautica takes place on. It is the largest passive creature and the largest creature overall in the game.

Much is unknown about the species: theories from the PDA in the game suggest that they travelled in small herds around the planet (much like the other passive Leviathan species, the Reefback, does now), surfacing to consume microorganisms en masse. Because of the outbreak of the Kharaa virus, food became extremely scarce for the species, causing their numbers to rapidly begin dwindling down to the point of extinction.

In the story
In the story of Subnautica, the protagonist, Riley Robinson, is contacted by the Sea Emperor via telepthy a few times upon doing certain things in the story. When exploring the alien facilities built by the Precursors, hints are spread out about a massive nearly-extinct leviathan species being kept deep down into the Active Lava Zone.

Once ariving at the Primary Containment Facility and gaining access to the Aquarium, the Sea Emperor leans on the platform, finally allowing the player to see it. Lore from the facility suggests the species and its babies were used to try and make a cure for the deadly Kharaa disease that had been plaguing many planets, killing over 14 billion organisms. However, no matter what the aliens tried, nothing worked to meet the hatching conditions for the Sea Emperor, and after the Kharaa was released by accident, they left the planet for good. It is also here that Riley learns that the Sea Emperor is responsible for keeping the ecosystem alive by having Peepers, small fish, transfer its weakened enzymes to temporary halt the disease and not have the entire planet die.

The Sea Emperor explains that the Precursors could not hear her telepathy and attempted to force the cure out of her. Her aquarium holds five of her eggs, and she soon explains that if Riley succeeds in hatching them, the babies will release a stronger enzyme that will cure the Kharaa disease permanently. Giving him the knowledge on how to fabricate the enzymes, she then entrusts him to fulfill her wish. After succeeding and all her eggs hatching, the babies swim off in a warpgate that Riley had activated earlier after their mother briefly played with them. As soon as they all leave, the Sea Emperor will collapse: she had forced herself to stay alive to see her young hatch. The Sea Emperor wishes Riley luck with his future, her babies indeed releasing the enzymes that can finally cure his disease, allowing him to de-activate the platform that shoots down incoming and leaving ships.

When Riley leaves the planet, the Sea Emperor contacts him one final time, thanking him for his efforts once again. It is at this point where she permanently dies, although in peace. Her grown juveniles can be seen swimming around in various spots. In the new game, Subnautica: Below Zero, they can also be found in the Lilypad Islands. Their purpose is not yet known.