is long ranged mage champion. He is very good on mid at early game and very helpful during teamfights. He is not typical mage-carry so if u like 1-combo KO champions u should better take or .

For Xerath i prefer standard mage runes:

Marks: Greater Mark of Magic Penetration;
Seals: Greater Seal of Scaling Mana Regeneration or Greater Seal of Ability Power;
Glyphs: Greater Glyph of Ability Power;
Quintessences: Greater Quintessence of Ability Power;

First i start with a Doran's Ring. It gives you some mana regen, ability power and health so you mustn't recall til you are 6. When you can afford another Doran's Ring and Boots go back to base and you will not have to recall for a long time. If you are good at stacking items just buy Mejai's Soulstealer, if not just don't waste your money better buy Rod of Ages. Try to buy Rabadon's Deathcap as fast as possible, it will give you some armor to. Zhonya's Hourglass is only item which give you armor and ability power, remember to use it's active (i usually put actives on 1,2,3 and use it by pressing hotkey). Void Staff isn't necessary because you have a lot of magic penetretion already, so if opponents don't have many magic resistance buy Will of the Ancients or Morello's Evil Tome. Don't forget about Rylai's Crystal Scepter you need it to your combo!

Xerath's best skill is Arcanopulse. Take it on first level to attack your enemies. Mage chains doesn't do much damage and has long cooldown but it marks your enemy and your next skill will stun him. Take it on level 2. remember that you can use it to run (use Mage chains, than Arcanopulse, stun your enemy and run). At level 3 take Locus of Power. Use combination Locus of Power and Arcanopulse every time and put your opponents health bar to 0 without taking any demage. Afer taking all skills maximize Arcanopulse. Arcane Barrage is very good and take it every time you can (you can use it to stun if you miss with Arcanopulse). Remember to use it 2 more times after first activation.

If there are some mistakes write a comment or send me private message i'll try to change it.