Oh. I was a littler surprised to see a Good Sombra but I think the couple really make sense and it is so heartbreaking. The story goes that Celestia, during at the beginning of her fallout with Luna, joined Starswirl The Bearded on a journey through a magical mirror to an alternate Canterlot. This mirror was revealed to be the prototype for the mirror that leads to Equestrian Girls's universe. There she meet the alternate version of King Sombra who is a good, generous and benevolent ruler of that world. In that world she also found out that the villain was her herself. The two fell deeply in love but was separated by the fabric of time and space. With each visit to Sombra's Canterlot, things started to go missing or repeat in Celestia's Canterlot like books borrowed by Starswirl in Sombra's world lead to mysterious disappearance of books in Celestia's world. Due to this Starswirl forbid anymore travelling between world to Celestia. However, she just banished Luna to the moon and needed more comfort than Starswirl can give. So she went back behind Starswirl's back. When Starswirl found out, he sealed the mirror but Celestia found a way to reopen it. But when Luna became good, the other Luna who supports Sombra turn evil and the Evil Celestia wanted to cross over to Celestia's Canterlot. Due to this, Celestia send the Main 6 plus Spike to Sombra's world since that world has no Elements of Harmony. But then they found out that to hurt Evil Celestia is to hurt their Celestia, so does sealing either one. In an attempt to save Celestia, Sombra absorbed all the evil energy from Evil Celestia and Evil Luna thus transforming into Evil Sombra however I can still see despite all that dark energy, his love for Celestia remained strong from how he cried at the end, bidding goodbye to the Celestia he loves while cradling her crown. The only minus from this story is the designs for Evil Celestia and Evil Luna that looked like a rip-off from Maleficent. Anyway, I'll be waiting for more inspiring work from you