The Zamorak bow has a special attack called Twin Shot, which uses 55% adrenaline. It deals 200% weapon damage. When Zamorak arrows are equipped, there's a chance to deal an extra 100% weapon damage (Fire spell damage).
There used to be a glitch in which the Grand Exchange stated that the bow could fire arrows up to dragon. It has now been confirmed in the FAQ thread in the Recent Events forum that the bow fires up to rune, in addition to firing Saradomin arrows, Guthix arrows, and Zamorak arrows.