Alien Queen

It flies, it only appears around the Vortex Pillar during the Lunar Events after the Lunatic Cultist falls, and no item summons it. The other way it arrives is by growing: an Alien Hornet left alive becomes one, so the pillar hands you queens you never saw spawn.

The goop it spits delivers the Distorted debuff, which is what kills you. Distorted bobs you up and down on its own and strips nearly all of your vertical control. Hooks still respond and the Rod of Discord still teleports. Flipping gravity with a Gravitation Potion or a Gravity Globe gives you a usable direction again. In Classic the goop only sometimes applies it; in Expert and Master every hit does, so treat it as permanent.

Its death leaves larvae, and those mature into hornets and then into fresh queens, so a careless clear rebuilds itself. Either finish them at once or use them: grown larvae give you kills that count against the pillar's shield from far enough out that you are not pulling new spawns. The same trick works with Star Cells at the Stardust Pillar.