The Deepsea Staff is a Hardmode summon weapon dropped by the Aquatic Scourge; it is a direct upgrade to the Seabound Staff. It summons an aquatic starfish that protects the player by performing dashes into enemies, dealing damage on contact.
Its best modifier is Ruthless. The Mythical modifier provides the widest array of stats bonuses, but these primarily affect the initial summon rather than the resulting minion. Additionally, Minions cannot deal critical hits. The only significant advantage a Mythical Deepsea Staff has over a Ruthless one is knockback.