For what reason Xerath's powers grow?
As I know it is never explained. There are many ways for that.
-
He gets used to his powers. He gains more and more control over them in time.
-
His sarcophagus gets weaker, so he can channel more power through his arcane body.
-
His arcane body gains more power with time. Let's say his body contains 100 magic power even as Unbound just after his Ascension, it becomes 110 with time. 120 even with more time. And it goes on (to a point he can't hold himself). I don't know how. Maybe it consumes magic around of him. It was said by Nasus to Taliyah to be careful or Xerath will rip out the precious magic she has. I don't know what it means.
-
He masters new types of magic. He could use a diverse spectrum of magic types even before his Ascension. His connection to magic never been that close, so I believe he is even more talented on magic than his human form.
Which is the most likely?