The Ur-Didact's goals are very different to that of the Forerunners as a whole. The Ur-Didact, following the events that led to the Gravemind twisting his mind and turning him into a puppet, believes that the Mantle belongs to Forerunners alone, and that the Composer is the key to imprisoning races like humanity who have wronged the Forerunners while still adhering to the tenets of the Mantle - preserving life. Humanity loses their biological form, but will go on to become Promethean Knights - the Ur-Didact's army, who will uphold the Mantle for the Forerunners by eliminating any suspect races who would seek to challenge Forerunner authority. Conversely, we have the other Forerunners who survived the activation of the Halos on the Lesser Ark. They swore never to interfere with the affairs of the galaxy again, realising the great wrongs that they had committed over the course of their time as the Mantle's custodians, and made peace with humanity - hoping that they would meet again one day as the brothers they were always meant to be. They reseeded the galaxy of life, and then went on the Great Journey. However, Chakas (who became 343 Guilty Spark) has found where they went and is on his way to find them having stolen a human ship and crew at the Ark. If they are going to be brought into the story again, they will more than likely be humanity's allies.