Theseus is trapped in some kind of loophole. It will be up to you to find out what is going on, and how to deal with it.
According to the developer, they wanted this action game “to be established on the uneven relationship between an “ordinary” man and a dreadful, invincible foe.”
Forge Reply states that the black substance of the body of the monster is a direct consequence of the approach of designing it to be more alien and disturbing.
The developer also states that one of the horn is broken, and suggest that this is related to the plot of the game. Of course they are not spoiling anything, so we will have to wait for launch for the answer.