Reaper

It spawns during a Solar Eclipse, and on current versions only once all three mechanical bosses are dead. That gate explains why an eclipse you handled comfortably earlier in Hardmode suddenly becomes a problem: the roster changes underneath you, and this is the enemy that changes it most. Nothing summons it directly, so the only way to see one is to be outside during the event or to force an eclipse yourself.

It runs the same behavior as a wraith with everything turned up. It drifts through solid ground, ignores lava completely, and moves fast enough that the eclipse spawn rate has several of them converging on you from different directions. Sealing yourself into a box achieves nothing, since walls are not an obstacle.

The one hard limit is vertical. It cannot climb more than a couple of tiles above solid ground, which is by far the most useful thing to know about it. A platform up in the air removes reapers from the eclipse entirely. That is not a complete answer, because vampires, deadly spheres and mothrons all fly properly and an eyezor can put a laser through you from the floor, but reapers are usually the ones actually killing you.

It takes knockback well, which is unusual for something that phases through terrain, so a fast weapon can hold one off the moment it emerges from a wall. Melee weapons whose swing continues after touching a block will connect with one while it is still buried, letting you start the exchange before it has arrived. The sickle on its sprite is the Death Sickle it carries. Unlike most of the eclipse lineup it is not referencing a specific film, just the figure everyone already knows.