Black stone arrows are tier 92 arrows. They are created by attaching black stone arrow tips to elder headless arrows at level 92 Fletching. The arrowheads are obtained as drops from the Shadow Reef. Before they can be fletched, the ability to do so must first be purchased for 350,000 Dungeoneering tokens from the Elite Dungeon Reward Shop ran by Bryll Thoksdottir at Daemonheim.

 

Black stone arrows have a passive effect that reduces the target's armour rating by 0.75% (to a maximum of 22.7 armour) for each attack, capping after 20 hits for 15% armour reduction (to a maximum of 454 armour). In most situations, this debuff does not wear off until the target dies. Some bosses clear all debuffs across phase transitions, including the black stone arrows.

 

Due to their passive ability, a black stone arrow is always consumed for every ability and auto attack performed, making them very expensive to use. It is recommended to switch to different ammunition once the target has been debuffed as much as the arrows allow.

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https://runescape.wiki/w/Black_stone_arrows