Mithril arrows were an item formerly obtained while playing Barbarian Assault. They could be received from the Attacker item machine, and could not be taken out of the minigame. Players using ranged had to use a different type of arrow to kill Penance Fighters and Penance Rangers, depending on which style was called out by the collector.
As players using Ranged were limited to using low-level gear as a result of this mechanic, Barbarian Assault was updated in 2018 to use four new types of arrows instead. These are twice as strong as Dragon arrows.