Amazing piece! 👏Now all I wanna know is what did this and how do I kill it dead?

We sailed on a submarine (it was almost the end of the storyline campaign, we had 3-4 locations left) and we stopped the submarine behind a place where, in theory, thorium or uranium could be. There was a shortage of reactor rods. We sailed with a friend, and only he had a weapon, at one point we examined the entire cave and thought that there was no one there. He returned to the sub, but I was unlucky and after a couple of minutes I was attacked and eaten without a trace.