Terraria (Wiki): Flamelash

The Flamelash is a magic weapon that fires a projectile that moves 90mph in exchange for mana rather than ammunition.

The Flamelash fires a controllable ball and can be controlled for up to one minute before dissipating and inflicting the On Fire! debuff (50%) for 4-8s. When the Use/Attack button is released, its projectile keeps its course until it hits something, and if enough space is available, it can pierce the enemy and then turn around to strike again. It will grow an increase in size after striking an enemy, but the second strike does not deal more damage (desktop, console, mobile). If fired up or down, the Flamelash will deviate in the direction the player is moving. The projectile will immediately home in on a nearby enemy, piercing it while maintaining its homing ability, and exploding on contact with another enemy or a block (desktop, console, mobile); through walls and doors, the projectile's explosion can harm opponents. The Flamelash's mana cost is only for initial casting, and players will regenerate mana while in control, and it will not cost more mana to maintain and direct it.

When used, the Flamelash has a low use time and a strong knockback which can be used to knock and pierce enemies away. It emits light particles that can fall deep into the ground and are useful for finding caverns, ores, overgrown caves, thorny bushes, and hornets. The Flamelash can be controlled in lava, but unlike Magic Missile, it cannot fire through water or honey. It also doesn't use mana while partially submerged in any liquid, including lava. 

Combat-wise, the Flamelash's strong knockback and speed make it effective against multiple enemies. It has the potential to function well in Hardmode as a magic weapon because it is similar to The Rainbow Rod. It can be considered an upgrade to Magic Missile.

The Flamelash is best modified with Mythical for overall stats improvement.