These are the runes I use for Xerath. They are pretty much a standard for any mage typically where you want to get as much AP as possible in order to burst people down. This holds particularly true for Xerath, where 15% of your AP is converted to armor.
I use typical caster masteries going 9/0/21 and grabbing all the standard caster necessities for masteries in the utility tree and offense tree.
Flash/Ignite or Ghost/Flash
These summoner spells are the ones I use most often. I prefer flash and ignite usually for early game kills and the escape/initiation of flash. Otherwise ghost is a viable replacement for ignite.
Skill Order 1-6:
The items I normally end up with for Xerath. Rabadon’s Deathcap, Will of the Ancients, Void Staff, Rylai’s Crystal Scepter, Sorcerer’s Shoes, and Zhonya’s Hourglass. Typically I start off the laning phase with a Doran’s Ring if I am feeling confident in not being harassed and being able to win trades. Otherwise I start off with boots and 3x health potions. On first back I will buy boots and another Doran’s Ring or 2 Doran’s Rings depending on start path. If I want to get really tanky health wise then I will rush a Catalyst of the Protector instead then turn that into a Rod of Ages. Otherwise I will farm and rush a Needlessly Large Rod and start building Rabadon’s Deathcap. Another situational build path for items would be to rush a Kage’s Lucky Pick for gold per 10, and start a rush build on Deathfire Grasp for the extra burst nuke. Will of the Ancients is my next item and if magic resist is needed I will grab Abyssal Scepter in place of Void Staff, which I grab typically mid to late game. Rylai’s Crystal Scepter is typically grabbed around the 30-40minute mark depending on how the game is going for more health if Rod of Ages is not grabbed for more CC utility. Zhonya’s Hourglass is my last item usually as it gives a nice immunity active if you are getting bursted hard quickly so you can live a bit longer.
During the laning phase you want to try and last hit as much as possible without taking too much harass from your opponent. You want to be conservative with your mana as well when using your Arcanopulse spell for harass. If possible line up your Q so it hits minions plus the enemy champion. Try to land your E and quickly follow up with a Q for a stun. This will allow you some free harassment with auto attacks. If you are level 4 try activating Locus of Power followed by a Q for a long ranged harass as the opponent runs back from you. This will give you the most zoning and free laning time as Xerath and really take advantage of his range. As Xerath hits 6 you want to combo your spells correctly when you unload for damage and burst on your opponent. Typically you want to land you E and follow up with a R as your ultimate is instant. Then Q, R, R, followed by a Locus of Power, Q for the finishing blow or to make sure they have to recall to heal. This combo should give you a kill if not bring them to low enough health that if you have ignite, it will finish them off. Another combination you can do is to bait your opponent by initiating up close with a Q, followed by E, R, R, R, Q. There are many combinations for Xerath that work, but I suggest using your ultimate for stunning as it is a instant cast ability and you can cast it 3 times for bursting the opponent down with a decent radius.