Mastery weapons are very power and designed for tier 3 character classes. After completing the 3rd Class Upgrade quest, your character becomes a tier 3 character class, also known as a "Master Level" character! You'll want to start using these items as soon as you've become master level. You can obtain mastery weapons from NPC Priest James. He has an item shop which offers items in exchange for Ruud, in Elveland. Ruud is mostly obtained from mini-games, such as Maze of Dimensions, after becoming a tier 3 character class.