Elf Copter

Nothing spawns it outside the Frost Moon, and you start that yourself with a Naughty Present at night once Plantera is down. The copters hold off until the sixth wave, so they arrive when the ground is already crowded and your attention is on the floor.

It flies through solid ground. Roofs, walls and the pillar you were sheltering behind mean nothing to it, so the arena tricks that shut out the ground half of a Frost Moon leave you completely open from above. The moment it has a clear line on you it opens up and keeps firing, and a group of them adds up fast.

Treat them as a priority rather than something to mop up later. They are fragile next to the heavy spawns of the same waves, and a homing weapon or anything with a wide arc will strip them out while you keep moving.

• The wave counter announces it as an Elf Helicopter, a name it carries nowhere else.
• A shooting animation for it sits unused in the game files.