The Covenant was sent here originally to farm recourses, once the UNSC caught wind of this they went to stop the covenant. Things didn't go so well...

you were abandoned by your team. They had thought you died. Your team left with the only way off this floating rock. Turns out the same thing happened to Dova 'Tugamee a Sangheili Ultra who was also left behind by his team by mistake. Now the two were trapped on this unnamed dangerous planet.