How do you manage to get a godlike being into a magic coffin, The lore would be better if instead of shuriman mages themselves trapping him (which would be impossible to do given the circumstances) Get a bunch of beings that have ascended throughout the lol universe to be the ones that trap him.
It only makes sense and would make the lore 10x more awesome. Imagine them watching over Xerath as he finishes the ascending ritual knowing that he cannot yet control his powers and therefore trap him until he can fully manage them without blowing the "lesser beings" to another place.
Since they are infinite, it would give xerath a chance to understand his power, because through the ascendants eye's he is nothing more than a newborn.

It is heavily implied that he does not truly control the magical energies. Directly after he truly gained them, it is no wonder that he was incapable of stopping the acolytes. This is further implied by one of the JoJ mailbag questions (sorry, don't remember which) in which someone said that Xerath was tampering with powers beyond his control. It was probably Ryze. Anyway, the coffin's remains limit his power. Even in the limited state, he has so little control over his powers. He can only tap into a tiny amount of said energy. His power is unfathomable, but the power he has that he is capable of controlling is relatively small.

he has had little time to explore his own potential, and his control of his awesome power is likely very little until he can explore them more (seeing as how lore never progresses, he will always be stuck in his sarcophagus wondering when or if he can control his power)
EDIT: after rereading his judgement, he states that his is merely a vessel, just projecting the raw power. while he appears to have little control, that is how he believes magic is supposed to be used. i will link his judgement for anyone who actually cares to read
http://na.leagueoflegends.com/news/l...agus-ascendant