When Sombra was born he was lucky not to have simply been left to die by the ponies who took one look at his coat color and thought him nothing but a monster, even his own mother. But his father had taken him to the princesses instead. He pressed his snout against the floor in the lowest bow he could; he begged the princesses to lift the curse on his son and change his fate. Celestia simply turned him away stating that the foals fate was sealed and the disharmonious magic tainting his coat would eat away at his mind till there was nothing left.
Luna, however thought otherwise, she believed that if he were to be raised properly he wouldn't succumb to his darkness; to be powerful enough to change his fate. Although Celestia was loudly against it and pursued her sister to leave the foal Luna wouldn't be bullied. She saw her own darkness mimicked in the colt and wouldn't leave it. She would have someone who understood and could fight the darkness with her.
Luna would change her own dark fate... even if it meant raising a monster.
*I haven't decided on all the characters that will be in the story so it will change accordingly*