Halo 3: ODST for Halo: The Master Chief Collection is slated to release on May, 29th 2015, as listed by Xbox.com.
The listing has since been taken off, but stated that the release date was May 29th, and that the game size will be 8.1 GB.
Halo 3: ODST will be the fifth Halo title to be included in Halo: The Master Chief Collection, joining each numbered Halo entry.
Halo 3: ODST will be free for those who played Halo: The Master Chief Collection between launch and December 19th, 2014.
This is 343 and Microsoft’s way of apologizing to those who purchased and played Halo: The Master Chief Collection during it’s very rocky launch, which was riddled with matchmaking issues among other calamities.
There is still no word on when those who do not qualify for the free download will be able to purchase the game, or what price they will have to pay. Expect a more detailed announcement regarding the game and how one goes about acquiring it soon.
