Eidolon Staff

The Eidolon Staff is a Hardmode wand dropped by the Lunatic Cultist. It fires either blue stars or an ice ball that acts like the Lunatic Cultist's. The ice ball, on contact with an enemy, spawn several more ice balls, which remain stationary on the enemy until it is killed.

Trivia

 * The name and appearance of this weapon is a reference to the Eidolon Wyrm.
 * The weapon was previously known as the "Stardust Staff", but was renamed and had its sprite replaced to avoid confusion.
 * If cast at a Target Dummy, the spawned ice ball will stationary on it, dealing damage to any enemy that comes in contact with it. The ice balls will keep refreshing on the target dummy until it is removed by a player, or the world is reloaded. Doing so on multiple target dummies can cause large amounts of lag due to the particle effects of the spell, it can lock up your game if you use it to fight bosses with multiple segments such as The Destroyer.
 * The tooltip of this weapon heavily implies that Eidolon Wyrms are transformed cultists.