Discord was once the Unicorn King.
He became Discord after he tried to use the unfinished spell that his court wizard, Star Swirl the Bearded, had created.
King Sombra is the older brother of Princess Platinum and son of the Unicorn King.
After the defeat of his father, Sombra was prepared to assume the throne only to grow enraged when the unicorns elected to instead be ruled alongside the pegasi and earth ponies by Celestia and Luna.
Hate fuelling his ambitions, Sombra began to dabble into the powers of the dark magic and after perfecting his new dark powers, he invade the nearby Crystal Empire in order to raise an army to overthrow the royal sisters.
Sombra, upon arriving the crystal empire used his new powers to corrupt the crystal kinghts and their ruler, the queen of the crystal empire.
The loving queen, so full of kindness and generosity, and her noble crystal knights, paragons of valor and honor, then became twisted and cracked creatures that felt nothing but the all-consuming need to fill the emptiness they now felt inside their hearts.