Microsoft and 343 Industries have announced that they are releasing Halo: Spartan Assault for Windows 8 and Windows Phone 8. Halo: Spartan Assault is expected to arrive this July and it will be the first Halo application that runs on the shared Windows core.
The Halo: Spartan Assault is a third-person shooter game that will retail for $6.99 across all of the platforms with 25 levels. The Gameplay feature on the game is a training series where class Spartan battles have been recreated.
If you buy the game for Windows Phone 8, it cannot be played on the PC. You will need to buy more than one copy to play the game on a different platform.
Halo: Spartan Assault achievements can be translated into Halo 4 gameplay and there will be better integration following the launch of the game. The game will have power-ups and in-app purchases. You cannot play head-to-head, but you can compare scores across a leader-board.