Princess Luna wants to return her nightmare guards to the dark realm from whence they were summoned. And there's just one pony who can help her with that: Trixie Luna Moon. With her Advanced Projects team, of course...

Since it's been a while, here's a quick recap for those new to the Advanced Projects series:

On an alternate branch of the G4 timeline, Twilight Sparkle never left Canterlot and is now an Acting Princess, with a very different group of close friends: Tempest Shadow, Starlight Glimmer, Trixie Luna Moon, Sunset Shimmer, and Moondancer -- plus Cheese Sandwich as their on-call party pony. And each of these has had a very different past, as a result of meeting Twilight Sparkle so early.

In particular, Trixie was once the Great and Powerful student and protégé of Princess Luna... while she was Nightmare Moon. Which has its consequences...

For more of this series, start with: A Canterlot Engagement.

Also, check out the entire series, beginning with Not Exactly Friends.