Pinkymod/Demon of the Abyss

The Demon of the Abyss is a Hardmode found within. It attempts to remains horizontal to the player while firing shadow orbs at them which inflict Shadowflame, if the player goes above or below them they gain a massive speed boost and charge at the player to deal contact damage until they are properly alligned once again.

Bestiary Entry
Pure mass of darkness, shadows, chaos, evil and regret. Volatile being of the Abyss, it is not tangible enough to be given a true form, but is powerful enough to manifest itself through the dimension as a cloud. They haunt the halls of the dungeon, and one should keep away from engaging them in vertical hallways, however few. They can catch up to you very fast that way.

You know it when you are in their presence. One of their most powerful abilities is that to slowly sap you of your sanity. It's not documented nor recorded, but any dungeon dwelver can tell you that. I'm pretty sure there are plenty of dwelvers who can confirm this, but they are probably hollowed skeletons cursed to walk that tomb for eternity amongst the rest.