Halo: The Dunes of 2560

After the destruction of the Ark, John-117, The Master Chief, returns on the Forward Unto Dawn, and Cortana & Thel 'Vadam, the Arbiter are MIA. The Gravemind survives, and a secretive Militant Organization makes a massive resurgence after decades of hiding. 8 years later, a Squad of Spartans and Sangheili find it to be their destiny to help Thel 'Vadam and Master Chief finish what they started. The catch is that a bizarre series of events leads them to going AWOL from their post on the UNSC base called The Dunes.

(Story takes place in an alternate reality based off the ending of Halo 3, anything Halo 4 and beyond is non-applicable.)