The Realm of the Primes is a mysterious dimension apparently inhabited by several fallen members of the original Thirteen. Some of them, like Micronus, can alter some things within the realm by their will. They can do things like changing, reshaping, or creating things for the realm as well.
Optimus Prime came to the Realm of the Thirteen following his sacrifice to restore life to Cybertron, as it was revealed in Out of Focus and Optimus revealed in W.W.O.D.? that the Primes would allow him to appear on material on Earth if Bumblebee was in dire need. Optimus then began undergoing training under the supervision of the other Primes in preparation for a great challenge that he would have to face on Earth. These tests included manifestations that the other Thirteen could apparently create at will, intended to help Optimus master the power of the Prime Decepticon Hunter. Optimus began trainning with Micronus as his mentor through a test that Micronus manifested a hill, hologram humans for Optimus to protect, and a robot foe with mini robots for Optimus to fight. After finishing this test, Optimus was still trainning under Micronus's supervision in Battlegrounds, Part 1. When Optimus completed this test, the realm became dark due to an evil force that was threatening to come. When Micronus brought Optimus to the other Primes, Optimus convinced them to send him to Earth, which Micronus explained that they must share a portion of their sparks. Optimus agreed and told them to begin the transfer, which resulted in Optimus getting a new upgrade and returning to Earth in a physical form.