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Mastery shows that you have completed all you can in your training.
To declare mastery your character must be level 100 and have all of
his/her weapons at 200. (To find out weapon levels type LEVEL)
When you declare mastery, you gain a powerful unique item specific to
your class, a special skill, 500,000 hp/mana/move, and a tier upgrade.
Be aware however; once you declare mastery, your mightrate is
increased significantly and pk is available at a greater level.