Celestia and Luna lied about being able to use the elements by themselves.

There were bearers for each of the six elements and Sombra was the bearer of the Magic of Friendship.

What happened you ask?

He lost faith in friendship and with it the ability to use his element after two of the element bearers (guess who) betrayed the others and began to conquer Equestria.

He fled to the Crystal Empire and created the Crystal Heart, based on his knowledge of how the Elements worked, to try and counteract their dark magic, or as they called it, "alicorn magic."

Due to a surprise attack he was unable to reach the Heart and had to resort to using dark magic himself to try and stop them but he was unable to stop them and was banished along with all of those still loyal to him, with the added insult of having their memories tampered with so that they would be unable to remember him upon their return.

4650516

their memories tampered with

Also, it's fanficton. Adhering to canon is kinda counter productive, don't you think?

4650522
I put "Story Idea" in the title.

4650500 I like the idea but it might be a fairly long story if done right

4650544 Is that a bad thing?

Also i like the idea

4650733 No but some writers don't like doing long stories

Now this is a story I wanna read!

4650516 and the fact befor that they don' t remember anything else. Heck if the writers of the show would do somthing about Sombra they can make him more of the victim then the crystal ponies.

4650837 Good Point

4654065
The writers are bound to bring back Chrysalis and Sombra at some point. Bound to. Maybe. Hopefully. Please?

4654682

4656746
you too haha