Moment of Kindness

Land discovered up north of the Crystal Empire, while Sombra remembers a moment from his colthood.

Past the Crystal Mountains, a unicorn discovers a crystal pony trapped during a snowstorm scheduled on that day. While King Sombra is alone in his throne room, he remembers what caused him to be the way he was.

Alternate Universe because Sombra wins and keeps the crystal ponies enslaved.