The "Magical Poison" ability from Enchanted Emerald Bolts.
Enchanted emerald bolts are mithril bolts tipped with enchanted emerald. Emerald bolts can be enchanted through the spell Enchant Crossbow Bolt (Emerald) by a player with level 27 Magic. The spell enchants 10 emerald bolts at a time.
Enchanted emerald bolts are enchanted with the Magical Poison enchantment. This makes the bolts count as being poisoned with extra-extra-extra strong poison (5 damage, even higher than p++ projectiles with a initial damage of 4), although they have an increased chance of poisoning their opponent.