Terraria (Wiki): Life Drain

The Life Drain is a Hardmode magic weapon that gives a Life Drain buff of +2.5HP/s to the player's existing health regeneration as long as it is hitting enemies in exchange for mana rather than ammunition.

The Life Drain heals more slowly than the Vampire Knives, but it applies a life regeneration buff while enemies are being damaged. The damaged enemies are afflicted with a special debuff that deals 5 damage per tick, and any enemy that is not immune to all debuffs is susceptible to its special debuff. The power of the buff increases for every two extra enemies in its area of effect and each targetted enemy releases a stream of red particles that flow toward the user. As long as enemies are still present, the Life Drain can still harm them through walls.

When used, as long as there is space between the player and the cursor, the Life Drain constantly deals damage to all enemies nearby when it is active. It does affect target dummies as if they were regular enemies, enabling players to recover faster after respawning without having to pay the Nurse.

Combat-wise, the Life Drain's low use time and damage output, make it a very effective weapon for crowd control and hordes of enemies. Even though it is not effective against single enemies, it can be useful against Pirate Invasions, Goblin, and Worm-type enemies since it counts each segment as an individual target. The Life Drain does indeed work against The Destroyer, but it does not affect the player's regeneration of life because the boss and its probes are immune to all debuffs. It is also helpful when fighting the Dungeon's Guardian, as the debuff will not affect its high defense.

The Life Drain is best modified with Mythical for overall stats improvement.