Demon

Standard Underworld residents. They spawn on their own the moment you are down there, with no summon and no gate beyond getting through the ash ceiling.

It throws a fan of spinning purple blades that start slow and pick up speed on the way in. That acceleration is why they connect: the dodge that looks safe on release is late by the time they arrive. You cannot shoot them down, but any solid tile stops them, so a roof protects you better than reflexes do. It swoops in and peels away rather than hovering, and it holds fire when something blocks the line, so mining hellstone from inside a tunnel is far safer than working open ash. The darkness it hands out is the real killer, since a blinded player cannot see where the lava edge is.

• Those blades are the Demon Scythe's attack, so an early drop hands you the weapon that was being used on you.
• Lava does not touch them, so the usual moat does nothing here.
• Watch for the rarer Voodoo Demon. Kill one over lava and its doll goes straight in, which starts the Wall of Flesh whether you were ready or not.