“This is honestly one of my favorite MLP ships, if not my absolute favorite.  
And to be clear, he isn’t the son of the Sombra from the season three opening that attacked the crystal empire. This is the child of King Sombra from the MLP comics. If anyone hasn’t read them, it’s issue 17, 18, 19, and 20, called Reflections. At it’s most basic, Celestia had been going through the mirror portal she helped Star Swirl the Bearded create and they found an alternate world where the villains were good and the good rulers were evil (i.e. Sombra was the kind ruler of Equestria and Chrysalis was the embodiment of Love) In the comic they fell in love but were ultimately permanently separated. I don’t want to spoil too much for anyone who hasn’t read it, so go do that now because it’s fantastic.
 
Siempre was the result of Celestia’s last visit to the alternate world, and all she has left of Sombra aside from their wish card and a glass shard of the mirror. He was born blind, and is every bit as kind as his father. His tone is always quiet and soothing, it has the effect of making those around him feel at ease and safe.  
Being the child of two world left him with some side effects, coming into too close of contact with anything that has caused a disturbance in the rifts between worlds begins to set him into a state like that of the evil shadow king Sombra, including Twilight Sparkle and Sunset Shimmer. The only person who has disturbed the rifts and doesn’t set him off is his mother, likely due to the genetic connection.  
He makes it his business to keep balance, be it personal affairs or in spacial rifts, attempting to mend the problems before they break out.”