Sombra always felt deeply guilty that Cepheus inherited his dark magic. He explained to him multiple times that it didn't make him a monster, that none of this was his fault, he's still the most precious foal in the whooooole world to his dear old dad!

Cepheus was very lonely growing up, being a royal kid made it hard to befriend other foals but being a cursed child made it a whole lot more complicated.
Fortunately for him, Mona (Celestial Monarch) was an outcast too due to her changeling heritage so the two royal kids just stayed together, they may have been called "freaks" by most ponies but at least they were freaks together.