Weapon Masteries and Archaic Powers are trainable abilities that are only effective against creatures. 

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Weapon Masteries (Warrior abilities)
> By selecting the Masteries option in your compendium, you will be able to activate one of the 5 powers (Tera, Fira, Aero, Blood or Doom).  Damaging or getting damaged by a creature while wearing a weapon will trigger special abilities depending on your power level.  

The trigger rate will depend on your weapons skill, arms lore skill, stamina level, current hit points, & your actual mastery power level.  You may only activate one mastery at once and you will be required to have arcane dusts in order to trigger abilities. Arcane dusts can be added to your character from the compendium and the talents menu.  

Archaic Powers (Mage abilities)
Killing monsters while wielding a spell book will grant the caster arcane points which can be used to enhance particular offensive spells, summons or to learn abilities exclusive to mages.

Killing any type of monster will give you XP depending on the monsters difficulty.