So my head canon for King Sombra, because the poor guy has no back story

The Crystal Princess of the time was Amethyst, a gentle ruler of Crystal Empire. She was granted Princess-hood by Celestia and Luna. She ruled for two hundred years, her subjects adored her. However, Amethyst fell in love with a unicorn stallion by the name of Dazzle. She could not bear to have him outlive her, so renounced her position of Princess. Celestia, found pity in her subject. She allowed her to remain Princess for as long as she lived, and her heir would continue the line.

Dazzle and Amethyst married, and Dazzle became the first Prince of the Crystal Empire. Shortly after, Amethyst gave birth to a pair of twins: Sombra and Luz. Luz was the first born, thus in line to inherit the crown after his father passed. Sombra was always jealous of Luz, often shunning his brother for his studies. He was dedicated to becoming more powerful than him. He hoped that if he surpassed his brother magically, he would gain the favor of his father and the crown.

His plan would fail, ultimately. Dazzle was disturbed by his son's fanatic energy and Luz proved more level headed and pony friendly.

Sombra became the first unicorn to master shadow travel and control. Essentially  it was another way of teleportation- but he soon learned to manipulate the shadows themselves. He began to weave spells of dark magic.

During the twins early adulthood, an illness swept through the Crystal Empire. Both Dazzle and Amethyst fell to this sickness, leaving Luz to accept the crown.

Sombra, of course, was not pleased. He challenged his brother outright. Luz refused to fight, attempting to placate the younger twin with promises of a co-ruled Empire. Sombra would not accept his offer, too enraptured by the idea of power. He banished his brother to the shadows, proving his power was greater. Thus, began Sombra's reign and descent into madness.