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Enchant Crossbow Bolt (Onyx) is a spell which enchants 10 Onyx bolts to create Onyx bolts (e), giving them the Life leech effect. This gives the bolt a chance to do a little extra damage and transfer some Life points back to the player, much like wearing full Guthan's armour. This does not work on the undead, likely due to the fact that they have no life to leech.
Completing the Seers' Village Elite Tasks increases the chance that the special effect occurs.
The spell may also be used on ascendri bolts to obtain ascendri bolts (e). These bolts also must be enchanted 10 at a time and gain the Deathmark special effect. This effect gives the player a chance to receive a deathmark status on each hit, which allows hits to provide an extra 1% adrenaline to the player for the next 15 seconds.