Disclamer: Long Post
Xerath being a "simple human" is not true at all. He was a great and probably the most powerful Magus of Shurima and he dedicated all his life to understand and amplify it. Also is not to disregard that Shurimans seemed like an advanced civilization when we are talking about magic since in Azir lore we find out that they had a "resurrecting pool" (which was probably reserved to the royalty).
It surely isn't a thing any1 dealing with magics could do. Carefully reading Xerath short story is clear that many emperors of Azir line were obsessed with it and there have been probably a lot of failures. The only succesful Ascensions we are aware of are the one of Susan (I'm sorry but I'm calling him like this since I discovered about it xD) and Renekton (being brothers they might have ascended in the same ritual) and the one that Xerath and Azir took.
About how it works we know nothing except that it requires the Sun Disc floating over the capital city of Shurima, the light of the Sun and other magic tricks. The ritual Xerath performed was way over the top since it had the following effects and aftereffects:
- It destroyed an entire city.
- It made it possible for Xerath to Ascend to a rather powerful being (superior to Susan and Renekton) and preserve the soul of Azir plus allowing him to Ascend and reacquire a physical body (with the help of the relics that still held the Ritual's power).
As a matter of "power levels" I think that Xerath and Azir are more powerful than Susan and Renekton with Xerath being on top of it. Now, let me explain... Azir, Susan and Renekton didn't have much power themselves except for being respectively a charismatic emperor, a wise seer and a strong warrior while Xerath was a Magus to begin with so he already had powers of his own. Add to this that if Xerath wanted his Ascension to be something worth to overthrow an empire where other Ascended guardians were still alive and kicking it had to grant him more power than usual. We don't know how much of the Ritual or about Xerath being already powerful/state of mind was the cause of his transformation (Long ago a Rioter confirmed me that Guardian of the Sands Xerath is what he would have been like if he Ascended while being loyal to Shurima so we cana ssume that the "energy being" thing is more bound to him being a mage already).
Yet we know that Xerath is able to see the forces holding the Universe together and he has a comprehension of the Cosmos which probably only celestial beings are aware of.
We'll have to wait for the answers but I have a good feeling about it.
