Theiaball was a planet that was a trojan of Earthball . It was roughly the size of Mars, and resided in Earthball's lagrange point. Theiaball is now currently in the form of Moonball (albiet with some of its mass in the Asteroid Belt), but its mind is currently in Earthball's core.
Even though the L4 and L3 lagrange points are relatively stable, possibly due to Theiaball's large mass relative to Earth and pertubations from Venusball, eventually she migrated closer to Earthball, resulting in a grazing collision. The ejecta from the impact gradually flattened into a ring around Earthball. It's possible that two moons actually formed rather than one, with the smaller one residing in one of Moonball 's lagrange points. As if to echo Theia's fate, tidal forces caused the smaller moon to merge with the Moonball.