Spike Ball

The Spike Ball is an indestructible enemy found in the Dungeon that swings around in a circle from a central pivot point (which also cannot be destroyed), and does damage to the player on contact. Separate Spike Balls can have differing chain lengths. This, along with Blazing Wheel, are the only two enemies that don't take damage. The spawn and despawn like any other enemy, meaning that when you first see one it may no longer be there if you get away from it.