Some lessons are too big for one letter. Others are so important that they need to be shared with ponies everywhere. When these two come together, Twilight Sparkle tries something new.

She writes a book.

Everypony remembers exactly what they were doing when they first found out about the griffin invasion. Many saw it as a growing shadow that threatened to overtake all of Equestria. To those in the towns and cities that would become the front lines, it was a terror made real. With Canterlot in flames and Princess Celestia injured, many wondered how anypony could make everything right again. I was among them.

But we did it. Together.

These are my experiences and lessons learned during The Great Griffin Invasion, and it’s not just my story. War is a dark affair, but we can always learn something from the worst of times. In fact, we have to. The lessons are too costly to let them go to waste. And maybe, with a little help from this book, we won’t ever forget.

-Princess Twilight Sparkle

(Valor is Magic is a story of war and values set between seasons two and three of My Little Pony: Friendship is Magic. As of the end of season three, the events of this story do not justify an AU tag.)

"Our life is a journey
through winter and night
we search for our way
in a sky without light..."

The old stories tell of the Mask of Lies, that entraps its wearer as certainly as it entraps everything around it. The worst lies of all are the ones we tell ourselves.

Spanish Translation: https://www.deviantart.com/spaniard-kiwi/art/Tan-facil-como-mentir-999963762

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?

Twilight Sparkle and her fellow Elements of Harmony have faced down supernatural foes of the highest order, but sometimes the deadliest danger can come from places one would least expect. As Twilight is forced to bear a devastating secret, her efforts to keep the Elements together may well tear them all apart.
The plotline of the thrilling Pony Psychology Series continues with an exploration of deception, despair and Discord.
This series takes place between Season Two and Season Three.
Secrets and Lies shares the Pony Psychology Series TVTropes page.

One by one, Twilight and her friends must come to terms with the consequences of being the Elements of Harmony.

These stories take place in the intervening time between Season One and Season Two,
with the exception of Ditzy Doo: Muffins, which takes place early in Season One.

The series continues in the sequel, Secrets and Lies.

Pony Psychology has a TV Tropes Page.

When Twilight plays with magic beyond her control, she finds herself and Rainbow stuck in a strange, horrible world. When they find out what happened, will they be able to get home? Will they even want to?