If wearing three pieces of Warpriest armour, there is a 5% chance that a 15% reduction occurs on abilities with cooldowns greater than 30 seconds. The chance that the ability activates is increased to 10% when five pieces of the armour is worn, or 20% when the full set is worn.
It is also dropped by Bandos minions in the God Wars Dungeon, and will make Bandos followers non-aggressive whilst in the Dungeon. Random pieces of the set are actually fairly easily obtainable, and for this reason, combined with the fact that the set has little practical worth, it is not uncommon for players to wear just one piece of the armour, and then simply destroy it after they've made their kill count. Once initially obtained, it can be reclaimed for a much cheaper fee than it would cost to buy it initially.