I'm calling it. That portal is not to another world but into Sombra's mind. This world and all that's in it is how Sombra sees Equestria. He sees himself as the good guy saving the world, but when a being as "pure" as the Real Celestia shows up he can't help but fall in love with her.

It will turn out that Starswirl made this connection and he banned Celestia from seeing Sombra again in fear that if the two Celestia were to meet Sombra's mind would break completely and they would be trapped, and with no Celestia to save Equestria and Sombra's mind shattered, Sombra would go on a rampage and destroy Equestria.