This is my xerath dominion build and i have been doing quite well with it

Standard spellcaster masteries nothing to out of the ordinary

ghost is a must to get between turrets faster, and i like to add in garrison because it works quite well with crystal scepter and even zhonyas hourglass, i used to use flash before hand but it doesnt save me enough in dominion, i guess you can go with heal too if you feel you need it but xerath is so amazing to cc with that if played well barely gets hit, you can also help with more cc by taking exhaust but once again its up to you i personally find ghost and garrison are all you need

People dont think the crit quintessences are a good idea but tell them to go suck a lemon, the rest is fine, he doesnt get oom to easily but i still like the mana regen runes just in case and you cant go wrong with magic pen and + ability

first 2 items tome and wand starting off good with +60 ap which is pretty damage inducing early on, next try and get boots as quickly as you can, followed by crystal scepter but if you want to go the more ap way go for void staff it could work well too but i like the movement slowing of crystal scepter it helps get out of sticky situations and makes them down their life away if they try to attack you at turret when you throw up that garrison too :) next i get zhonyas because hes a bit squishy so in group fights you will most likely get singled out first plus once again if they attack at your turret zhonyas/garrison is a nice mix too +100 ap is awesome and then finally just get as much ap as you can till match is over, usually only have time for rabadons but if the match looks like it will last quite a bit longer you can throw in a cd reduction item before rabadons like morellos evil tome or maybe even switch out sorcerer shoes for boots of lucidity