Years before the start of the fairy tale Friendship Is Magic, Princess Celestia had a husband called Prince Sunset. When Princess Luna was Nightmare Moon, and she had returned, she had locked away Princess Celestia into the sun. Prince Sunset was soon pinned in Nightmare Moon's castle before Apple Jack, Twighlight Sparkle, Rainbow Dash, Pinkie Pie, Rarity, Spike and Fluttershy got there. Watching the short lived battle and seeing Nightmare Moon become Princess Luna, and his wife freed. But when Princess Celestia looked for him, Princess Luna kept the secret he was in a secret chamber only she can acess. Understanding her sister, Celestia gave a nod and just left what she thought was her true love. Princess Luna made Prince Sunset swear he wouldn't leave the next day at the parade that signified Princess Luna's return, and he did promise with all his heart he wouldn't leave.
He didn't.
That night, when Princess Luna came home, she couldn't help it but to become Nightmare Moon again.
Going outragous, she killed him.
When she went back to normal, she let out a shriek of sadness. She thanked herself the walls were made out of rock and brick. If it were crystals and glass like Celestia, there was a huge percentage her sister would hear her.
Frantac, Princess Luna waited for her guards to fall asleep, then ran out with the dead body and buried it in Celestia's least favorite place that she only visited once as a foal.
The wolf cave.
After fleeing the diseased wolves, she flew in through her empty window of her royal bedroom and slept that night.
Celestia visited the next day.
So she would be in less trouble, Luna told the truth...