Mastery 4: Your units with Flying get Killer.
Card sounds may differ from in-game as they get updated.
Flying - Flying units can only be blocked by other flying units, but can block ground units normally. They can also be directly attacked by Killer units and Relic Weapons.
Killer - A unit with Killer can make one special attack directly against an enemy unit. This can be done the turn it's played, and can target any unit, regardless of Flying, Aegis, or other special abilities.
Mastery - All damage dealt by units with Mastery count towards its Mastery ability. When its Mastery number is met, it uses its Mastery ability.