Crimson Axe

It hovers in place, spins to build a charge, then lunges in a straight line, passing through whatever terrain sits between the two of you. The lunge is the only part that can hurt you, and any knockback cancels it and puts the thing back to spinning. Hit it consistently and it never lands an attack at all. Let up for a second and it crosses the room instantly, which is why sealing yourself into a pocket achieves nothing except removing your warning.

Arkhalis and Terragrim shut it down outright. Neither wins on damage. They win by holding a constant hitbox around your character, so the axe spends the whole encounter being bumped back into another wind up and never reaches you. Any rapid swing with knockback on it does a weaker version of the same thing.

There is a fair chance of being cursed on contact, and cursed means no items whatsoever: no weapon, no potion, no hook. In the underground Crimson, where these show up alongside everything else the biome throws at you, a few seconds of being unable to fight back is more dangerous than the hit that caused it.

It carries spawn rules most enemies do not. A town NPC in the area switches it off. So does standing at a tile backed by safe background wall, which describes nearly every room a player builds. It also checks that you have moved, jumped or used something in the last few seconds, so a parked character generates none at all. A Shadow Candle, held or placed nearby, waives every one of those conditions at once. The ground rule is looser than people assume: it only needs crimson material underneath, so crimstone or red ice that has crept into an old tunnel will produce them even where the biome does not officially register.

The sprite is a retired version of the one the Blood Lust Cluster used to wear. The spin then charge pattern lines up with the flying sword familiar from Symphony of the Night closely enough that the influence is hard to argue with.