The player doesn't know it yet, but the parasite is part of another story in which it attacked the planet of Nnnn. The Nnegrals had to evacuate, so they escaped with their "Generation Ship" and fled. The parasite had stowed away, however, so the crew was mostly zombies. The absence of a crew made the ship seem abandoned, so Arkus boarded to raid the treasure.
When Arkus walks to the middle of the room, the boss appears out of nowhere. The boss has no special defenses, so the player can hurt it whenever it appears (that includes the very beginning). Three Zombie Nnegrals will rise from the dead to attack the player. When all three have been been defeated in rapid succession, (they revive after a short period of time) the parasite comes out and will be vulnerable. Repeat as needed.