Flow Invader

It appears during the Lunar Events, in the airspace around the Stardust Pillar, and nowhere else. That puts it behind the Lunatic Cultist and everything gating him. It spawns as part of the pillar's own population, so there is nothing to summon and no way to meet one outside the event.

It arrives with a small entourage circling it, and those orbiters are its ammunition. It flings them at you one at a time and grows replacements steadily, so a settled one keeps up close to a shot a second. Catch it early, while it is still assembling the first set, and there is almost nothing to dodge.

It also throws everything it is still holding the instant it dies, so a fully stocked one killed at close range is a burst you have to be ready for. That alone is a reason to fight them at a distance where you can see the shots coming rather than parking on top of one.

The projectiles explode on contact with terrain and the blast covers real area, so a tunnel or a low roof turns every near miss into a hit. The orbiters can clip a block and detonate while they are still circling, which catches out anyone who backs into a wall.

It tries to hold position directly above you. Running in a straight line just tows it along overhead with a clean firing angle, so circle instead and keep it correcting. It cannot drop down through platforms either, which means a platform layer over your arena pins it at a height you choose.

The orbiters are projectiles rather than creatures, although the files also contain a fully drawn miniature version of the enemy that was never used.