The Harpy Staff is a summon weapon. It spawns a Harpy to fire feathers at your enemies. Like other minions, summoned Harpies are invincible and follow the player for an unlimited amount of time, unless the player dies, summons a replacement minion, or cancels the buff.
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 Harpy Staff has over a Ruthless one is knockback.