Aurelion Sol's backstory feels really incomplete. One of the reasons that Riot cancelled Ao Shin was because they had many questions regarding him. Many of them are related to lore. "When we examined Ao Shin with these new lessons - Why was he in Runeterra? What were his motivations? - we unearthed a fresh thematic that really set us off running."
That's because it is literally incomplete. They haven't released his extended lore or his color text, both are standard parts of lore nowadays. Just look at 
's lore. That is what a complete lore page looks like.
It's a short story,his long lore(that will likely be on the targon page and his champion page when he gets released) will likely explain it a bit more.
I think its very likely that the mysterious city atop of Mt.Targon is the race that tricked Aurelion into servitude.
