When first thrown, the Boomerang will always pierce enemies, however when it returns it will no longer have piercing and will bounce off enemies, dealing damage. After bouncing 6 times or the enemies die, it is able to reach the player.
The blueprint for the Boomerang is located in High Peak Castle, next to the exit to the Throne Room. It is locked behind a door that requires the 3rd Castle Key to enter. For this the player must defeat all the Elites found in the three colored rooms throughout the level. Additionally, the player must have the Ram Rune and the Spider Rune, or the Homunculus Rune to reach the blueprint.
Synergies
Boomerang can be used with most Tactics weapons as it can deal passive damage as well as applying the on hit effects of its affixes frequently, such as Oil or Poison.
Boomerang's ammo is affected by the mutation Ammo.
Networking drastically increases biome clearing speed and efficiency.
Notes
The damage dealt by Boomerang is not reduced when it is used from backpack via Acrobatipack.
Trivia
This weapon used to be unable to pierce enemies, but would inflict critical damage when returning to the player.