After the Human Covenant War, and the countless billions of trillions of lives lost during the conflict and subsequent repairing and money being funneled out for fixing all that could be salvaged during the great conflict, the UEG & the newly reactivated & reformed CAA, dispatches a UNSC Naval Expeditionary Fleet to a far off small, untamed planet cluster, to try their hand at colonization again. It's located on the far outer rim of the Orion Arm and controlled UNSC space. It was overlooked during the Domus Diaspora, and spared the terrible fate of being glassed during the Human Covenant war. The Expedition Fleet is doing this under orders to search for planets to colonize, and colonize them if they come across any. And luckily for them, their exploits weren't in vain. Because they found one, a very small, but special planet, sitting cozy within the habitable zone of the lonely star.

Unbeknownst to them, there was a reason they planet wasn't colonized before, because somebody beat them to it already.