This story is a sequel to Spiders and Magic: Avengers Assemble
A world of heroes would ideally be a good thing for Equestria, but with the heroes came the villains, and after so many villain showdowns and multi-dimensional troubles from other worlds outside Marvel and Equestria, order needed to be restored to maintain the harmony Princess Celestia worked for many years to achieve. With the help of Tony Stark, the two design a method in which the hero work would be properly moderated and harmony can be maintained.
But over time ideologies began to clash as Steve Rogers began to doubt the Accords laid out by Tony and in time would stage a rebellion to fix Equestria of it's newest grasp. Friendships will be tested, allies will become enemies as a battle breaks out between two groups, those who are aligned with The Accords, and those against it. This is war.