The Deadly Sphere Staff is a Hardmode, post-Plantera minion-summoning weapon that summons a Deadly Sphere minion to fight on the player's behalf.
The Deadly Sphere flies continuously and charges at enemies to inflict damage. When charging, it will not be able to pass through walls, but otherwise it can move through blocks to get closer to the player.
The Deadly Sphere has several visual forms, which it periodically transforms between after it attacks 3 times, though this has no functional effect.
The Deadly Spheres are a possible reference to the movie Phantasm.