Hello everybody! This guide is meant to help you people understanding how to play with the champion Xerath.
First and foremost: this is just a GUIDE. You don't need to follow it to the letter, or if you don't like it, simply don't follow it.
Second: This guide was made with the information i gathered while using the champion, only.

Pros:

Incredibly long reach spells;
Extreme burst damage;
Three times usable ulti (Arcane Barrage);
Not as squishy as other AP Carries;
MAGIC PENETRATION.

- Awesome taunt animation (Ctrl + 2) -

Cons:

Easy target;
Weak while burst is in cooldown;

Starting up the game, get your Arcanopulse (Q), followed by the Mage Chains (E), followed by your Locus of Power (W).
After getting all these three, invest in upgrading both your Arcanopulse and Mage Chains.
Whenever it's possible, get/upgrade your Arcane Barrage (R).


At the begining of the game, you should use your Arcanopulse basically for harrassing your opponent, for Xerath's reach will allow him to hit his skill, without being targetable by your foe.

After lvl 2 get your Mage Chains, for that is the starter skill of your burst combo. After using that skill on your enemy, do your best to hit him/her with your Arcanopulse before 3 seconds come to pass. That will stun your opponent for 1.5 seconds.

After reaching lvl 3 get the Locus of Power. This skill will grant you magic penetration and will increase your skill's reach. Being aware of that, you should use this skill as a pre-combo (it's not lethal if you don't use it before your burst, but it's much better if you do).
This skill is also good for chasing an scaping foe with low HP, for it gives you improved movement speed for a short while after it goes off.

Whenever possible, get/improve your Arcane barrage, for it IS your burst. It can be used three times, if in quick repetition, before it goes on cooldown. It is also a skill you should favor when you have many opponents against you, since it deals damage in area.

In case you simply don't want to read all the skills' descriptions and such, here's the combo you should use:

W, E, R, R, R, (W)*, Q.


(W)* = In case your foes are running away and you are still planted to the ground, use W again to be able to move and get a movement speed bonus, than snipe your scaping enemy with Q. I don't recomend this move, but sometimes it can be useful.