This story ties in with Shisno Chronicles: Friendship is Magic.

For a week now, Chronicle and the rest of the Keepers of the Elements of Harmony have been suffering nightmares for a week without explanation. There is an evil force tormenting his friends and for once he can do nothing to stop them. And then things get worse, when Rarity gets kidnapped by the evil force, Twilight Sparkle immediately sends word to Princess Celestia, who arrives shortly with Princess Luna.

From what Luna tells them, Rarity's abductor was the same force that had converted her to Nightmare Moon. When she was defeated by the Elements of Harmony, the evil force—the Nightmare Forces—retreated to the Nightmare Dreamscape on the moon and lay in wait…until now.

Can Chronicle, the rest of his friends, and Luna stop them before it's too late?

[Author's Note: Since this apparently takes place within Season 3, this story's hiatus will be much longer than I thought, given that Trixie, Babs Seed, and Lightning Dust appear.]