According to the press release:
The Overdrive Map Pack includes two new multiplayer maps, Abaddon and Sky Dock. Amidst the magma craters of Abaddon, protect a power cell facility built on a scalding energy source. Sky Dock is a crucial military base and space port nestled in the frozen tundra. Each map is playable in both multiplayer and offline play for all game modes, which includes Swarm, Conquest, and the recently community-unlocked Assault game mode.
The DLC will cost 320 Microsoft Points ($3.99 USD).