Slimer

Nothing summons this one. It spawns by itself once a world has entered Hardmode, anywhere the corruption has spread, both on the surface and in the corrupt caverns below. A crimson world never produces one, because the crimson has no winged slime of its own, so a whole playthrough can pass without you meeting it.

In the air it behaves like a bat, drifting in loose arcs and swooping in rather than tracking you cleanly. When its health runs out the wings come off and the body drops. That is not a death animation. What lands is a live slime that hops at you under ordinary slime rules, with its own health left to chew through, and the first time it happens most people have already turned around and walked away.

The handover gives it no immunity window, which piercing weapons take advantage of for free. A shot that strips the wings can come back around and connect with the grounded body on the same pass, so anything that bounces, lingers or passes through effectively skips the second half of the fight. Once it is down there it also weighs almost nothing. Its knockback resistance is close to the lowest in the game, so a heavy weapon sends it across the room and it spends the rest of its life hopping back toward you.

The grounded form looks exactly like a normal Corrupt Slime while being weaker in every respect, which cuts both ways. You can over commit to something harmless, or read a genuine corrupt slime as one you have already softened up. Royal Gel removes the whole problem, since both forms ignore you while it is equipped. The wings themselves look like a pair of demon wings and it drops nothing of the sort.