I am currently playing terraria Internum for the first time and when i first fought the king slime this fight was normal (with the gem), i came back after a few days and didnt install any new mods and this is what my king slime looked like with completely different attacks. I did end up defeating it but it was stuck in a death animation so it was invincible. I think whatever mod this is from is conflicting with infernum and i have no clue what it is. I tried searching for it but nothing came up.
by adept_chainsaw