This is my first build on MobaFire. I was inspired to create this after a game I played and someone was asking me what my masteries, build, etc. was for Xerath. So here it is, ENJOY!

This build is based on creating a brutal AP carry Xerath. I very rarely get to complete the entire build due to the other team surrendering because I'm fed and destroying everything.

For runes:
Marks - Magic penetration (Greater Mark of Insight)
::These are pretty self-explanitory. Aimed at giving you an edge of magic pen on enemy champs.

Seals - 6 flat mana regen (Greater Seal of Replenishment) and 3 mana regen/level (Greater Seal of Clarity)
::Some people may wonder why I split between the two. Granted come level 8 the mp5/level seals are going to give you almost 3 times the amount of mana regeneration as the flat seals do. However, once you are level 18 mana is generally not an issue. This is aimed at giving you more mp5 in the laning phase for farming up your cs using you Q while still having mana to pick up kills.

Glyphs - Ability power/level (Greater Glyph of Force)
::These are also pretty self-explanatory. I go with AP/level rather than CD redux, because after my item build I am generally right around the 40% cap for cooldown reduction anyways. So, I'd rather have the extra AP.

Quints - Flat ability power (Greater Quintessence of Potency)
::Same explanation as the glyphs; flat AP quints will give you a nice edge at the start.

The masteries are pretty straight forward with 22-0-8. I do that rather than 21-0-9, because of the bonus AP you get while ignite is on CD with Summoner's Wrath.

The items are pretty typical for an AP champion.
I start the game with a Doran's Ring and try to stay in lane long enough to where when I return to base I can purchase level 1 boots, blasting wand, and a sapphire crystal.
On my 2nd trip to base (assuming I haven't died before this) I will upgrade my boots to ionian boots of lucidity, finish catalyst protector, and if I have enough finish my Rod of Ages.
You should have these 3 items by the 20 minute mark at the very latest - I usually shoot for getting my Rod of Ages by the 15-17 minute mark even if it means waiting to upgrade my boots til my next trip back. The sooner the better on RoA so it can start building your AP, HP, and mana.
After I get my first 3 core items I start on building my Rabadons Deathcap. Some people may like to throw in a Tear of the Goddess to build an Archangel's Staff after the RDC, but I generally do not get Archangel's. However, this is certainly an option for you.
After RDC is completed I work towards a Rylai's Crystal Sceptor. This item is beautiful with Xerath. Once you hit with your E this will give them a little slow to help you better land your stun with your Q or your Ult.
Depending on how I am doing I occassionally grab a Fiendish Codex before I get started on Rylai's to give me some extra CD reduction; it just depends on how the game is playing out. If I go this route I will get a Fiendish Codex, then Rylai's, then finish the Morello's Evil Tomb. After finishing morello's you are sitting at 35% CD redux between that and your boots, and maxing out at 40% whenever you have a blue buff, thus the reason why I avoid cooldown reduction glyphs.
So now your build is sitting at boots, dorans, rod of ages, RDC, rylai's, and morello's.
Your last item will generally depend on the way the game is going. If you decided to go with a tear of the goddess you will have sold your dorans already and would now complete your archangel's.
If you feel like you need that last 5% of cooldown reduction, and want another nice nuke to go with it I would recommend a Deathfire Grasp.
If you are playing an AD heavy team I would suggest a Zhonya's Hourglass. With Xerath's passive already boosting your armor; a Zhonya's will give you even more of an edge against AD champs and a nice 2 second invulnerability when things get really hairy.
Lastly, if you find that you are up against a tanky team stacking a bunch of MR I would recommend going with a void staff to cut through it. That stacked on your magic pen marks plus the bonus magic pen from using your W this will let you melt through magic resist like butter.

For Summmoner Spells I go with flash and ignite. Flash is self-explanatory; it can be used to escape a sticky situation or to jump in on your opponent to pick up a kill.
Ignite I use to pick up kills often and to limit their heal on themselves if they say use a potion during a fight or have a support champ nearby. Also, with the summoner's wrath mastery, when ignite is on cooldown you get a little AP bonus.

Early game farming is the most important aspect of Xerath. He has great farming ability with using his Q, and his attack animation is short and simple making him easy to last hit with. When you see a group of low health minions use your Q to pick up 2 or 3 CS. If you see just one by himself use your auto attack to last hit him. Try to save your E for using on an enemy champion to follow up with your Q to stun him.