The Rune Song is a legendary sword that suffers no durability loss with use. It also gives significant health regeneration per hit and has a chance to instantly kill any enemy with less health than the player.
This unique effect used to be abusable with massively boosted player health by consuming many Giant Mushrooms or Amber Larva, before the change to single use health increase consumables. However, Rune Song is one of the most difficult to acquire items in the game.
Right click to channel crafting while holding the Rune Parchment, with all other ingredients in inventory.