hmm... idk how much power xerath has. or what this "cosmic energy" is made up of and how much of it he has.
sand can be made up of many different things. most of it is silica.
actually i think someone like azir might prove to be especially powerful against xerath if he can control sand like that.
the original idea that nasus and renekton had with the sarcophagus was good.. it really depends on what kind of energy both champions are limited to...
But i imagine Azir just creating a huge sphere of sand, and encasing xerath in it...and any of xerath's energy that acts against it would fuse the sand, encasing him in a permanent sarcophagus.
lots of things depend on it really, but that's kind of how i imagine the fight going.
i'm leaning towards azir's side, mainly because xerath is an a-hole, and his power isn't very well defined.
idk he can probably just break out of it though. i don't like vs battles tbh