xerath is one of the champions that vary according to game play. Xerath is a very powerfull burst mage while being a little more tanky due to his passive. With this build you will be able to be a tanky burst caster. you can take out 2 enemys easy if you get familiar with this build. this may not look like the best build for a burst mage but i say that is is one of the best out there.

This rune set lets xerath dominate early game. you have one of the best burst set ups possible. the AP and magic penetration let you destroy through the game.

The masteries are different from some but they allow you the bonus AP witht the 21 in attack and the 9 in utility allow level and gold to get your build faster.

I love this build with xerath the opening item always with him is a dorans ring. As soon as you get your first 1100 gold grab Sorcers shoes by level 6. As soon as you have them get 860 for your blasting wand to start your archangle staff. finish up you archangle because this will give you mana and ap. Then after that build a Rabadons deathcap, when this is built you will dominate every lane you enter. As soon as i build this i move to another Archangle staff. Your not the tankiest yet so stay back and harrass with locus of power. when you have enough you should pick up giants belt and turn it into rylais scepter allowing for slows and health. When you have this you are a tank OP bully you can take out champs no problem. the next item helps takeing out magic resist so you are unstopable. Locus of powers range with spells will dominate team fights

haveing your Q with your W at level 2 allows very good harrassment. the early game runes and items will let you dominate. focus your Q by level 9 and your E by level 13. Dont forget your W at level 2 though or this will affect your game very badly. Get your alt at level 6,11 and 16 like all champs

Exaust helps early game for first blood and flash can help you catch up or to escape. other optional spells are clarity and teloport. no clarvoyance or ignite

Farm with your Q shoot the minion wave 1 time then finish it with 1 more. But early game try to last him minions and poke the enemy champs with your Q

Pros

good burst
very durable late and mid game
very fun

Cons
hard to play at first
mana hungry at first
different than all other champs

I really like Xerath and this build works for me and my friends it destroys and will always detroy. so go get 400 kills with lightning bolt.