Hi there! This is my first build,so.. Let's say one thing what is characteristic for Xerah:

Xerath is a long range mage champion with a couple of near tricks. In addition to deal tons of damage with his abilities he also can bomarding enemyes for long range.

For runes,im take magic penetration marks, ability power per level seals, ability power per level glyphs, and flat ability power quintessentces.

This setup is aimed at giving me early lane game dominance,using Arcanopulse to push arround my opponents while also scraling me into late game with tons of AP.

My masteries are 9/0/21, taking magic penetration in offence and making sure to take increased cooldown reduction

For items,im open with Doran's ring,then i grab Sorrcerer's shoes. After grabing sorrcerer's shoes I immiediately grab Rabadon's Deathcap. With my core build done, I add utility and damage with Ryaly's Crystal Scepter and Will of The Ancients.
I finish out my build with Morello's Evil Tome for cooldown reduction and Void Staff if my opponents start to get more magic resist.

Xerath's passive is Ascended form Ascended form.Xerath feeds on arcane power, making him increasingly resilient to physical harm. He converts a percentage of his Ability Power into Armor.

Xerath's first ability is Arcanopulse Fires a long-range beam of energy, dealing magic damage to all targets hit.

Xerath's second ability is Locus of power Xerath anchors himself to a source of magical power, increasing the range of all spells and granting him bonus % Magic Penetration. Although he is immobilized while Locus of Power is active, his movement speed increases upon disabling the ability.

Xerath's third ability is Mage chains Deals magic damage to an enemy and marks them with Unstable Magic. The next spell Xerath strikes this enemy with stuns them.

Arcane Barrage is Xerath's ultimate. Calls down a blast of arcane energy, dealing magic damage to all enemies in an area. May be cast up to three times before going on cooldown.

I Usualy grab:

Exaust-Nice way to slow your enemy deal tons of damage with combo of Locus of power, Mage Chains and Arcanopulce.

Flash- Great for chacing enemies. Im always grab that one.

Good choices:

Ghoust- Great Altenative for Flash! It's help for chacing alot.

Ignite- Dont let your enemies to run away! Use flash and ignite and watch how they burns!

Well,that is the end of my poor,new build. Vote and comment!