Cyberra
The Destroyer
Rather than quitting and reloading which takes forever, I just let him kill me if I'm not up for the fight.
Moon Lord wasn't actually summoned. Something honked off the cultists and the cultist archers were running back and forth tracking on me, but I blocked off the mouth of the dungeon entrance a while back so they couldn't actually reach me and none of them died. They were just mad at me for some reason. Never even saw the Lunatic Cultist. I chose to book it and go poke around a different world.
Uhm, is there a fix for maps blacking out? I went to the world I generated just after the release of 1.3, in which I'd done a lot of exploring and farming, and both the minimap and the large map were all blacked out again.