@H i s o k a Given how (a) The name of the "Zombie King {blank}" boss was glitchy and then made hidden to MC, (b) how this Zombie King boss appeared right at the entrance and started attacking immediately (acting very strangely) and how this dungeon is not normal and (c) how this weird, new snake guy looks, talks and acts a lot like a human (even dresses like one)...
I think it's safe to conclude that: (1) This snake guy is responsible for the strange behavior of the boss and this dungeon. He probably controls both. (2) This snake guy used to be human, but is one of the "human traitors" that MC has dealt with in the past.
Remember, back in chapter 1 or so, where we got a bit of the backstory of MC with a flashback of how human traitors have ruined Earth, making humanity lose the war against the monster invastion?