PH40k: Luna Heresy

The Equestrian Imperium had three founders: The Sisters Celestia and Luna, and Void, who was the First Dragon-Master of Mars. Very little is known about each one except for this:
In foreshadowing of the Heresy, the two sisters were at first bitter rivals for global control of Holy Terra after the Dark Age of Technology had ended. Luna was the head of a massive network of guerilla warriors, while Celestia was the general of the best army on the planet. The only thing the two sisters had in common was that they were refugees from the same ruined Psyker-Pony village. Thankfully, Void the Dragon was an industrial genius that had united Mars under his wings and used his military to stop the warring sisters before they could destroy each other and Holy Terra. Void convinced Celestia and Luna to work together and bring ponies and dragons across the galaxy back into order. Thus they began the first crusade.
As they conquered and annexed planet after planet, Luna and Celestia encountered other ponies that were great heroes and warriors like themselves, and they recruited them as leaders or "Primares" of the Space Mareines. The number of Primares would eventually reach 20, each sister had ten close friends.
The Equestrian Crusaders discovered diffent species of Xeno-Animals on different worlds. Many of these animals had fallen under the power of the Chaos Gods and they could move through Warp-Portals and survive in Warp-Space. Void and Celestia saw these Chaos-Xenos as evil, but Luna saw them as being free and even more powerful. Knowing what her friends would think, Luna kept her interest in Chaos a secret amongst her closest Primares and that led to their fall.
The truth wasn't revealed until the three founders seperated to run different aspects of their great empire. Luna would continue frontline expansion whith 15 of the Primares, Celestia would supervise the internal affairs of the empire with 5 Primares, while Void would lead research and developement of technology back on Mars.
When Void had created a device which could permanantly banish demonic species from entering real space, he thought that it would be a great weapon for Luna. When Void brought the device to Luna, he discovered that she was being physically corrupted by Chaos and he confronted her alone. The arguement between the two literally raged for days until Luna stabbed him with her mutated and venom filled horn. As Void staggered back and began to die, Luna walked away and broadcasted to the Primares under her command that Void had been betrayed by his own guards.  Before Luna could give an order, Void jammed her signal and said what had actually happend. He implored the Primares that if they didn't want the Imperium or Celestia to be destroyed, they would protect his guards and his device from Luna. Enraged, Luna killed Void with her bolter, and gave the order to exterminate Voids guard and the Primares who would protect them.
The 10 Primares and Mareine Chapters that were friends with Luna turned against the five that were still loyal to Celestia. Despite being outnumbered, those Loyalist Primares and Space Mareines succeded in saving the guards and collecting Voids body. But Luna was able to damage Voids banishment device by ripping out it's mysterious power source as it was being hauled away by loyalists.
As the survivors of Luna's betrayal fled inward to the empire to warn Celestia, Luna's Heresy became known to all. Void was solemnly laid to rest in a tomb on Mars, and Celestia wept.
Civil war engulfed the Equestrian Empire. Armies, planets, and even Primares died. The war eventually reached it's conclusion when Luna launched a final strike against Celestia. They fought on the surface of the moon orbiting Holy Terra, and when it became clear that neither sister could gain the advantage, Celestia armed herself with Voids banishment device. Luna mocked her sister, but stopped when Celestine explained that the banishment device could use Psyker abilities as an energy source. Celestine only fired the device once and it pulled both Luna and the Moon into Warp-Space and charged them with energy that kept them from entering Real-Space. In doing so, Celestine had her own powers drained to a bare minimum and she was charged with energy that would keep her from ever entering Warp-Space.

Thus Celestia still rules over an empire that she helped build but may never practically see. And every 100 years, she makes the long journey to Mars and the tomb of Void.