Magic Missile

Magic Missile is one of the numerous Magic Weapons that are found in Dungeon Chests throughout Terraria. Activating the wand casts a multi-colored stream of sparks, which will accurately follow the player's cursor across the screen in any direction, provided it doesn't collide with a solid wall or an enemy. However, the 'missile' will detonate automatically if the player releases the attack key (by default, the Left Mouse Button) It is high in damage, but expensive in Mana. The cost is for the initial casting of the missile; maintaining and directing the missile does not cost additional mana.

http://www.youtube.com/watch?v=5RX0iJLVO84 - Video guide

Trivia

 * Magic Missile is based on the spell of the same name from the Dungeons and Dragons Pencil-and-Paper RPG. In 'D&D', Magic Missile does not miss, unless some sort of solid object stands between the caster and their target, which likely inspired the Terraria spell's 'remote-control' behavior.