Disclaimer: I do not own any of the My Little Pony: Friendship is Magic characters. They are owned by Hasbro.
An alternate ending for those that enjoy them.
Eight ponies proceeded down the hall of Canterlot Tower. The bearers of the Elements of Harmony and the princesses of Equestria held their heads high as they made their way past the stained glassed windows of the heroes of the past. Each of them wore a stoic expression, fighting back tears that threatened to spill. There had been enough sorrow; now was the time to honor one of their own.
Princess Celestia and Princess Luna, who had returned after an extensive pilgrimage throughout the land to reacquaint herself with the ponies she helped rule, stopped in front of a window covered up by a red curtain. The princesses parted and stood on either side. Simultaneously, their horns lit up and drew back the curtain to reveal the window underneath.
It was as grand as all the other windows in the tower. At its top were depicted the bearers of harmony, their bodies aglow with power. At the bottom lay Discord, his misshapen body fallen in defeat. And in the center stood a blue unicorn, her body acting as a conduit for the elements to strike down their common foe.
Perched on the sill of the window, lay a star covered magician's hat.
"My friends," Celestia announced. "Let us honor today a pony that sacrificed herself so that we may all live happily. This window is a promise that her noble cause will not be forgotten. May her soul rest in peace."
The princesses bowed to the window. After a moment of gazing at the glass structure, the bearers did the same. All except one.
There came the sound of a magical discharge, followed by an ear piercing shattering of glass.
A collective gasp escaped from the ponies as they scrambled away from the window to avoid its falling pieces. Once the danger had passed, seven pairs of shocked eyes stared at where the window used to be. Only a few shards of glass remained in the frame. The rest was strewn across the carpeted floor. Then, as one, they tracked the pony responsible as she stepped forward, her hooves crunching glass as she went.
"T-Twilight?" Applejack stuttered in disbelief. "Why?"
Twilight Sparkle didn't answer. She stopped just below the remnants of the window, the sun's rays striking her at full force now that there was nothing to impede them. Twilight remained silent for a full minute before she found her voice.
"This...this isn't...right," she began, struggling against the emotions building within her. "I...I made a promise. I promised that she'd be here, in this room, seeing this window. All of us. Together." Twilight lowered her head and shut her eyes tightly. The tears building up finally spilled forth and fell to the floor.
Suddenly, Twilight whirled around and faced the gathered ponies. Her muzzle was set in the most determined expression they had ever seen. "I'm going to keep that promise! Somehow, someway, I'm going to find a way to separate Trixie from Discord. She'll be here with us to celebrate like she should be. I swear to all of you, this will not be the end of her story."