Blade Staff

The Blade Staff is a Hardmode summon weapon that is dropped from Enchanted Swords. When summoned, a miniature Flying Knife spawns above your head and follows you around. When an enemy is nearby, it flies to the enemy and goes back and forth like a Stardust Dragon. It has relatively low damage, but ignores large amounts of enemy defense.

Trivia

 * The Blade Staff has the lowest base damage out of all the minion summon weapons in the game, but the low damage is easily compensated by it's ability to ignore enemy defense.