Mere moments away from being killed, the Didacts takes one last chance at preventing Humanity from wielding the Mantle of Responsibility. And in doing so the Didact uses a device so deadly and unpredictable that the Forerunners swore to never use it again. Although for the Didact, he could care less.

The Xyzox. A mysterious race with possession of technologies beyond human imagination, have wrecked havoc across the entirety of the Milky Way Galaxy for over a century. With the Xyzox about to be crushed under the force of a united galaxy, they're forced to resort to their final solution, retreat into an alternative universe. One that will be forever changed upon the Xyzox arrival