Giant Bat

There is nothing to summon and nothing to trigger. The moment a world turns hardmode these take over the cavern layer and push the Cave Bat almost out of it entirely. Every dive from then on is against these instead, a sharper jump than most people expect out of one altar.

The contact damage is survivable. Reversed controls in a cavern are not. Get confused over a lava pool, or while backing away from something worse, and your own inputs kill you rather than the bat. Break line of sight, stop moving, and wait it out instead of fighting the controls. The effect also runs longer on the harder difficulties, roughly doubling by expert.

Expert worlds add a second problem: a bite that keeps rolling fresh ailments onto you long after the bat that gave it to you is dead. On a long trip down that matters more than any single hit it lands.

The accessory that makes you immune to confusion comes off this bat. Grinding them for a while solves the exact problem they create.