Source

Discord has realized the Mane 6's friendship is too strong for him to face head on. He goes back in time to destroy it completely, thrusting Twilight and Spike into an adventure that spans their lifetimes. After discovering that Discord's changes in the past are causing chaos in the present, the only way for Twilight and Spike to fix Equestria is to go back to the future!

*Author's Note*

Be warned, there is going to be some sad and dark stuff in this story, but I don't think it's enough to warrant the use of either tag.