Its best modifier is Ruthless. The Mythical modifier provides the widest array of stat bonuses, but these primarily affect the initial summon rather than the resulting minion. Additionally, minions cannot deal critical hits. The only lasting advantage a Mythical Deadly Sphere Staff has over a Ruthless one is knockback.
Summoned Deadly Sphere minions appear to have a shorter-than-average lock-on range (about 40 tiles) compared to many other summoned minions that can detect enemies off-screen.
The minions can be summoned anywhere on the screen, even on solid tiles, and give off light, making the weapon useful for cave exploration.
The Deadly Sphere has several visual forms. It periodically rotates between these after having attacked three times. This has no functional effect.
The staff may appear or be reforged with the Deadly modifier, resulting in the name "Deadly Deadly Sphere Staff". A similar trick can be done with the Lucky Horseshoe, the Lucky Coin, and the Arcane Flower.
Like the enemy that drops it, this item is a reference to the 1979 American science fantasy horror film Phantasm.
One of the designs for the Deadly Sphere resembles a TIE Advanced, a fictional starfighter in the American epic space-opera media franchise Star Wars. However, it is actually a visual reference to a variant of the sphere seen in the 1988 American science fantasy action-horror film Phantasm II, the sequel to Phantasm.