Mastery
Mastery stats are used to track and reward long-term war operation activity. There are both Spy Mastery and Wizard Mastery stats/rankings.
Acquisition
Successful war spy black ops award Spy Mastery. Successful war or black-op spells award Wizard Mastery. Chaos League operations between members also award mastery when they are treated as war operations.
The amount gained is based on the mastery difference between you and the target, plus up to 4 additional mastery from the target's defensive SPA or WPA. Equal mastery starts from 4 mastery before the target-ratio bonus; targets with higher mastery are worth more, and targets with lower mastery are worth less. The target can also lose mastery when the attacker has at least 100 mastery.
Reward
Spy Mastery increases spy strength recovery and reduces spy losses on failed operations, scaling up to +2% recovery and -50% spy losses at 1,000 Spy Mastery.
Wizard Mastery increases wizard strength recovery and reduces spell mana costs, scaling up to +2% recovery and -20% spell costs at 1,000 Wizard Mastery.