There isn't a paradox
let me explain...
The rules of time travel are simple
1) You must not have a body
2) You can only travel through time in 1 direction
3) Whilst traveling through the past, you may only go somewhere you have already been
4) You must eventually return to your own time
5) You cannot change the events of the past
That out of the way, Ansem, Seeker of Darkness discovered that he had the ability to travel through time... Some time after he learned about it, he developed a plan and went back in time to let YMX know about it (the nortification plan)...
SoD didn't change anything when he went back in time mind you, he only let YMX know about the plans (He would have continued down the EXACT same path regardless) With that knowledge given, SoD went back to his own time and the events of KH1 occurred... After SoD took over Riku's body, he commandeered the body and Riku was expelled out of his own body, at that point (when SoD entered into Riku) he no longer had the ability to time travel
Back to YMX, this guy then continued on with how everything would have happened, at some point he traveled forward in time grabbing all the norts along the way and bringing them to TWTNW
(btw, YMX (Young Master Xehanort) is MX (Master Xehanort) back during his youth... They are literally the exact same person, just at different points in time)
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Make since?
