Eater of Souls

They are the Corruption's standard flier, the opposite number to the Crimera in a Crimson world, and they need no event or boss behind them. Ebonstone and corrupt grass both breed them, which means the surface patches trickle a few out while the chasms pour them. Three sizes exist and the game rolls one at random each time, so an encounter can be a nuisance or a real problem depending on what turns up. Shaking an ebonwood tree has a small chance of dropping the smallest variant on your head, which catches people out during ordinary wood runs.

One alone is nothing. They rarely arrive alone. They hang back and circle just outside your reach, then commit one at a time, and while you deal with the one that dove the rest are still orbiting. Reach and piercing beat raw damage here. A spear, a Cactus Sword, or the Enchanted Boomerang that hits again on the way back all do more work than a stronger weapon with a single short swing.

This is the enemy that teaches new players to leave the Corruption alone until they have real gear. Solid defense, resistance to being shoved around and heavy contact damage add up quickly when half a dozen are on you at once, and there is nowhere to back into on open corrupt ground. Expert makes it noticeably worse: they cut most of the circling and fly straight at you with far better tracking, which removes the breathing room the pattern normally gives you.

Chasms are the source, and any block will do to cap them. Sealing a chasm mouth traps whatever spawns below it, and the surface keeps producing them at a much lower rate, which is the difference between clearing a corrupt strip and being driven off it. Doing this before you go looking for shadow orbs is worth the few minutes it costs.