The eponymous feature of the DLC is the Mastery Progression: you will gain experience for every game of Core Defense you play. With every level up, you will gain a mastery point, which you can spend in mastery pages unlocking bonuses like the following ...
Wave Preview: See which mobs will spawn in the next wave
Angry Mobs: Enrage mobs for an extra bonus reward
Corner Core: Moves the core into the bottom left corner
... and of course more walls, various stat bonuses, better wall stacks, better roll chances, you name it! You can create multiple mastery pages for different setups and then choose one of these pages when starting a game.
Supercharge
With the DLC, you can toggle SUPERCHARGE mode when starting games to activate one additional difficulty modifier chosen at random from the following options:
Shootout: Only shooting mobs are spawned.
Friendly Fire: Your units will hit each other.
Rocketeer: Only Rockets are spawned.
Speedy: Mobs are weaker but faster.
Bossy: Every wave is a boss wave.
Disabled: No ability rewards.
Fragile: Don't let a single mob escape.
Duality: Two spawners, two cores.
Chaos: Random spawner locations.
Unity: One suffers, all suffer.
Wait there's more yet!
Ten more difficulty levels: Overload 21-30
After beating Overload 25, you can freely choose your primary color
