Board Thread:Questions and Answers/@comment-195.46.241.230-20170217182510/@comment-79.212.46.9-20170217185920

I'd say it was definitely a Reaper Leviathan.

I had an occurence a few days ago, where that thing spawned right in my face (it just popped into vision), instantly killing me and destroying my already damaged Seamoth. Reaper Leviathans are still very bugged. They phase through the terrain just like that and if the water is too shallow, they might also decide to take a swim in the air or "fall asleep" for a few seconds. Since you said yourself that you were getting lost because of the darkness, I believe you've been maneuvering close to or above the Kelp Forest.

My hypothesis is, that you died from collision damage due to wrong calculation of the collision vector caused by glitched spawn or movement of a Reaper Leviathan.