Holy snickers of heavens, that's sweet. Glad to see you're alive!
This is the Lunar Boss. It's Yrimir's spaceship. It has 10,000,000,000 health and it one shots you with that huge cannon. The reason why it is the Lunar boss is because the spaceship blows up the Moon as an attack, showering moon rock onto your world.
Were you wrong about the whole brain of cthulu joke, lol. There hasn't been a single day i stopped checking for summoner playthrough!
Damn, how do you guys build such good things? All my stuff is rubbish