Halo 3: ODST was originally released on the Xbox 260 in 2009. The game took the series to its first non-Master Chief oriented game where players took the helm of several Helljumpers, or orbital drop-shock troopers [ODST]. Through the eyes of each of the squad you see the mayhem that took place in the city of New Mombasa after a Covenant attack.
This was also the first game in the series not to feature a basis around player versus player mode but instead a Firefight mode. Firefight featured an onslaught of covenant rounds in which you had limited lives to defend the area. For those still seeking to play a player versus player mode, received a disc containing Halo 3’s multiplayer.
If you played the Master Chief Collection prior to December 19th on the Xbox One. You might be receiving a message with a game code to redeem the game for free. For everyone else you can purchase the game for $5.
Source: Halo Waypoint