Reaper Leviathan

The Reaper Leviathan is a large, hostile creature.

The Reaper seems to move around its prey for a few seconds before attacking. Currently the Reaper doesn't harm the cyclops, though it has enough strength to push it.

It can currently be spawned in experimental mode using the command SPAWN REAPERLEVIATHAN.

If fins and head protrusions are included, the Reaper Leviathan currently is about 2.5 times as high and about 2 times as long as the Cyclops.

The Reaper spawns around the Aurora, usually alone. This creature spawns rarely because it is so dangerous.

Abilities
The Reaper Leviathan is able to instantly kill any players it attacks outside of their submarine. An early version of the Leviathan's player-kill animation can be seen here. The Reaper Leviathan is also able to grab the Seamoth and shake it around. Unmanned Seamoths inside a Cyclops can also be attacked, as well as the player while piloting. This is most likely a bug. Early Seamoth-attacking animations can be found here.

As of 4/3/15 the Reaper's animations are implemented. If you approach it with the Seamoth, it will grab the Seamoth and shake it around, doing 2-5% damage before letting go. This will most likely be tuned up in the future, as the damage is currently very underwhelming.