After the end of Halo 3, Master Chief and Cortana ends up drifting towards a Forerunner planet, but it's not Requiem, they receive aid from an unexpected source and are sent their way back to Earth, but a rare glitch happens where they end up in the land of zombies,

Spartan-B312 survived Reach, but not without a great cost, he proceeded to fight with through the rest of the war with John-117 and in the latter parts, Thel Vadam' the Arbiter. Peace now reigns in the UEG/Sangheili controlled space. In the midst of this, a certain event happens to our Spartan during a test for the new Mark X series of MJOLNIR Powered Assault Armor

After the end of the great war, things become more tolerable for the belligerents and willful survivors of the Human-Covenant war as the new generation has looked forward to joint peace, prosperity and technological advancement. During these times, an Infinity-class ship is sent to investigate a Forerunner artifact, a now commonly increasing sight, and is sent to a trip unprepared.