It has been two hundred years since the eve of Nightmare Moon’s birth, and two hundred since the foundings of the Kingdom of the Sun, and the Kingdom of the Moon. Now, these two solar superpowers fight a cold war with one another… Supplied by free merchants, the technocratic Changelings, and pirate fleets. The Kingdom of the moon, comprised of six orders of knights, are suddenly thrown into civil war when the Order Argent, an order who had dedicated itself to the eradication of those they perceive as mutants and criminals, is suddenly the target of a brand new order that has risen from oppression. Meanwhile, four ponies, Marble Merlon, Cursed Symphony, Brick Bastion, and White Rose, find the fate of the entire system placed at their hooves.
 
While certainly not a crossover, it takes inspiration from Warhammer 40K, Red vs Blue, and Warframe, while filled with medieval and fantasy themes.
 
Link.