Harpoon

The Harpoon is a unique Ranged Weapon that auto-fires an arcing projectile that is connected to the player via a chain, and returns after reaching a maximum distance or impacting. Its physics are similar to a Bow and Arrow, though it returns before each repeat firing. It has a relatively long range, high Knockback, and is highly affected by gravity. One Harpoon is usually dropped at some point during each Goblin Army invasion, which is the only way to obtain one.

The Harpoon has no cooldown period, its auto-fire rate dependent only on the distance to its impact, meaning that an enemy or object in close range will cause a faster auto-fire rate. The projectile also deals damage and knockback to multiple enemies on its return trip, similar to a Flail or Boomerang, making it effective for crowd control.

Trivia

 * The Harpoon acts as Flails used to before the 1.0.6 update, except with increased range.
 * Even though Goblins drop it, they never use it in battle. The same goes for Spiky Balls.

Tips

 * The Harpoon is useful on all pre-Hardmode bosses. The Eater of Worlds can be killed rather quickly simply by holding the fire button at the point where it is emerging from the ground. Skeletron takes a reasonable amount of damage per hit, and the Eye of Cthulhu fight is made rather easy. You don't even need to make an arena, though a small Wood Platform in midair does help by allowing you to fire at Bosses beneath you.
 * As the Harpoon is the only source of Ranged Damage that doesn't consume ammo, (Apart from the Piranha Gun.) this weapon makes a decent (albeit weak later in the game) backup weapon for characters specializing in ranged weapons and armor.
 * When surrounded by weaker enemies, it may be a good idea to fire OVER the crowd, hitting them all on the return trip, rather than firing straight in front of you and only hitting one enemy at at time.
 * The Harpoon remains a relatively useful early-Hardmode weapon, serving as a way to defend yourself until you get access to Hardmode weapons and spells, like the Golden Shower.

History
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