Called the "Summer Celebration", 343 Industries has some fairly big news pertaining to past Halo installments. Guardians was confirmed to be receiving a 4K update for the Xbox One X. In addition, the remaining four Halo titles that are not now backwards compatible were announced to be joining the program before the end of the year. This includes Halo 3, Halo 3: ODST, Halo 4, and Halo: Combat Evolved Anniversary. Noticeably absent from the list was Halo 5 and it looks like we now know why. In addition, all Halo games that released on Xbox 360 will become backward compatible later this year for Xbox One.
More details about the Xbox One X features will be revealed at a later date.
Longtime fans have even more reason to celebrate. In an intriguing twist, anybody who owns those games either digitally or on disc will be able to play either online or via LAN against players on Xbox 360 consoles-which is rare for games in the Xbox back-compat program. Cross Lan play between Xbox One and Xbox 360 will also be available. The team here at 343 always had a desire to bring players the best Halo 5: Guardians experience for Xbox One X, but we weren't quite ready to make the announcement during the E3 time frame. This is about as good as anyone could hope for, short of concrete news about future games.
Microsoft is also making all of the map packs for these four 360 games free to keep the community together.