Tower: Beam of Ruination - Items League of Legends
Attach, then recast to fire a damaging beam from a turret to your cursor.
First Cast: Attach a Slayer Beam to the target turret that can be fired 3 times.
Next Three Casts: Fires the attached beam towards your cursor, dealing 30/level + 30% of the hit target's maximum health (20% damage to minions) in magic damage to all targets in a line.
Beam will last 15 seconds, or until it has been fired 3 times.