(This is a one-shot)

A thousand years ago, the elements of harmony had been created. A thousand years ago, the two alicorn sisters used them together, and with their powers, encased Discord in stone. A thousand years ago, the elements of harmony locked Sombra away into the depths of ice, losing the Crystal kingdom in the process. A thousand years ago, Nightmare Moon became corrupt and was banished into her own object of desire. Nine hundred and ninety-eight years ago, Celestia met five of the six Rumors of Harm. She destroyed four. One month ago, Sombra was destroyed as well. Sombra was a Rumor of Harm: the Element of Fear. The final remnant, corrupted forever by the taint, has returned, and its powers have increased demonically after the destruction of its counterparts.
The princesses had six Elements to oppose the six Rumors, which kept balance, but removing just one grain of sand is enough to cause imbalance, and imbalance brings destruction and disharmony.