So, I've had Enigma as one of the mods I use, and just made it to the point of fighting Etheria. Long story short, after killing it the first time I guessed the result was a clue to fight it again, and I guess that was right since the fight was different, but when I died Etheria didn't despawn, and just flew around killing everything, including me a few times, while shooting out countless rocks and lagging the game. I do have a mod that speeds up the respawn time, but I'm also not meant to respawn while a boss is alive, which has worked with modded bosses so far, so I'm not sure what's going on.
It did eventually despawn after killing me four or five times, but the colour shift is kind of annoying, and enemies... Don't seem to exist? Which persists even after quitting and loading in again. Is this intended, or has something gone wrong?
edit: I managed to resolve the problem by cheating and being immortal so I could kill Etheria again. Which I'd normally not want to do, but with no enemies spawning I couldn't really do anything to get stronger and prepare to fight it again properly.