Arron Dominion
Achievement Hunter
The exclusive Halo title originally for Windows Phone/Windows 8 (which migrated to the Xbox platforms) is also receiving a release on Steam for all Windows platforms. It will be on sale for $5, will have 30 missions with steam achievements/leaderboards, and is set roughly in between the 3rd and 4th Halo. This version will support both controller use and keyboard/mouse.
Source:
http://www.gamespot.com/articles/halo-franchise-makes-its-debut-on-steam-next-month/1100-6418592/