During the fight with the gargoyle on the top of the church, position yourself behind the enemy boss and attack the tail. Once you've lopped off the tail (Monster Hunter style) and claimed it, you will receive the Gargoyle's Tail Axe (check your inventory). The weapon comes with 93 attack and can be upgraded for additional damage. Note that the axe requires 14 strength and 14 dexterity to wield.