Shadow was born as a unicorn to an earth pony father, and a unicorn mother. His father was unmatched in strength through out equestria at the time, and his mother was the leading expert on elemental magic, she even learned how to channel the energies of the elements through her to strengthen her magic.
When Shadow was born, he inherited his mother’s ability to channel the elements though his own body, but he did not have full control, so he had to be fitted with inhibitors to regulate how much flowed out, or in.
Shadow also inherited some of his father’s strength, and all of his father’s Iron Will to keep ‘fighting&#