After aiding Chrysalis in invading Canterlot again (and being defeated), it's up to Luna as another one of the redeemed to find out the reason for Discord's betrayal. Alternate timeline running from beginning of episode 3x10.

Ravenous diamond dogs. A savage stallion and a lost mare. A sanctuary in an oncoming storm. A prince of alicorn blood. Sacrifices, debts, honor, and oaths. Political games and war. Ten long, beautiful years, and all the stories that fall within them.

Somepony has come to Ponyville looking for Twilight Sparkle and intends to find her before dark forces are able to harm her and her family.