I know it doesn't say exactly in lore (or at least what I read) but how strong do you think Xerath's mind manipulation abilities are? We saw him control an entire army from his Capital. Does this mean he can go to Noxus and take control of whatever armies they have and use it to his favour in Shurima?
As far as i remember, and i could be wrong, Xerath manipulates people, but not in a magical way of mind control. He is just very good at convincing people.
I think its mostly just a "big bad guy, people scared to stand up to him" generic plothole.
Xerath can pretty much just vaporize anyone who he doesn't like and wipe out families / destroying homes from far away.
As for renekton, you can`t really blame xerath, Renekton may be ascended but he is still a biological creature that was stuck inside that tomb with xerath for such a long time that he would have gone insane regardless, xerath was just there to abuse that and without a biological body he couldn't be tortured by the perception of time (although he still understands time).
