Ascension - What is it?
So, now long after the Shurima Event, I got to the question: what exactly IS "Ascension"?
All we know is that it is supposed to be an empowerment ritual used in old shuriman times while using the energy of the sun. But how does it work? Are there downsides to it? If it was so awesome, why wasn't every Shuriman an ascended? Are there possible side effects? And, of course, could ascension be replicated by "modern" methods? (aka Hextech/Techmaturgy).
I will be talking about six champions here:

Yes, I know Cassipaia is cursed and not ascended, but I will get to her and her curse since it might be an interesting topic that might be connected to this. And about Twisted Fate... I'll explain that later.
So, about the first three. At first it seems like ascension doesn't turn you into a generic magical super-human and there seems to be some individuality. But how does it work? Does a popup plop up in your mind the moment you ascend, asking you to choose appearance and distrubute skill points? Not very likely. It seems like ascension turns you into what you think you are. Sounds weird, but let me explain.
It's very likely that the Shuriman Ascension technology was shrouded in mystery and religion, even for the shuriman people. So maybe some thought that it would turn people into gods or make them equal to gods somehow. Now, let's imagine that the highest god resembles a bird-man. So what form comes first to mind if an emperor thinks of becoming "a god"? That bird-man-god. Azir views himself as the "Emperor of the Sands" - what is his power? Sand control.
It's very likely that the same thing happened during Nasus' and Renekton's ascenseion. During the process they identified themself with their gods who were closest to their profession and turned into something that was similiar to how they imagined them.
Xerath on the other hand... is what could be called "true ascension" - because of his desire for ultimate freedom he just got that: being free of everything, even of a mortal body. He didn't want to become something "similiar to a god", he just wanted to be "free" - quite fitting for a former slave.
Now, why did I list Cassipaia before? Actually it's quite simple. If the appearances of ascended/half-human beings show their "true being", how would the people of Shurima react to someone with the body of a snake - an animal that has probably quite some bad reputation there?
The cell where Xerath and Renekton where held was secured with a defense mechanism, made to scare people away.* "If you open this tomb and free the evil inside, all shall know that YOU were the one who betrayed Shurima and sided with the traitor"*. I can imagine the snake curse being some form of "highest possible punshiment" in shuriman law, since it's basicially an excommunication from society - who would help someone branded as a traitor?
Ok, and now to Twisted Fate.
Empowerment through ritual? Check. Ritual conducted by a guy who's some kind of magic-pro? Check. Said pro is called Xavier Rath, what sounds awefully close to "Xerath"? Check. Possibility for a being like Xerath to use illusion magic and blend in with modern society? VERY HIGH.
OK, but why would Xerath be interested in replicating the ascension ritual? One possibility could be the hunger for more power, trying to ascend even further. Another one could be "I lead the world to absolute freedom, EVERYONE SHALL ASCEND AND BECOME IMMORTAL LIGHTENING BLOBS LIKE ME!" - we should keep in mind that we have no idea what Xerath is planing since he left the tomb. Or maybe he's just playing a bit of "mad scientist", we can't know that.
But why isn't Twisted Fate a half animal or some form of sentinent blue light? It's simple, because Twisted Fate never asked of being turned into that. All he wanted was magic, and that's what he got. He might be immortal though, but how would he know? For that he would needs his powers for some decades to realize he doesn't age.
Tl;dr: Ascension seems to be bound to the subjective expectations to the result of the ascending person. Also: Beware of people with suspicious names.
Your opinions on this?