Hello, I'm Pyros and I've been playing Xerath since he came out, testing lots upon lots of builds. Xerath is a very nice, but a bit hard to master, champ. He can dish out a tremondous amount of damage at an astounding range - it's simply music to my ears, when I'm being called noob, just cause I W and Q, while they are one second from teleport home :). This build is by no means perfect, It's just the way I find Xerath to be most potent.

Xerath can be a valuable asset for a good team or a complete failure, who can't land even one spell.

Pros:
- Best range of all casters
- Great damage
- Cool voice and lots of awesome
- Can be harder to kill for ad carries, thanks to Ascended Form
- Has potency to be the best in-game mid in the right hands

Cons:
- Squishy
- Mana hungry, if you spam abilities too much
- Skills are hard to hit if you don't have any experience at skillshoting and barrage
- Skills also take a fraction of second to launch, especialy W, so might get a little hard in close quarter combat

Most valid for Xerath are:
Flash - Perfect for running and closing in for that last Q
Teleport - Get to turrets, tp to a nice, cozy ward in bush

Less valid:
Clarity - Might be usefull early on, but late game you even forget you have it, and it's useless if you build tear
Cleanse - you are squishy and cc vulnerable, but it's a waste late game, beacuse usually you are far from cc range, nuclear bombing from afar

Ignite and Exhaust - Why not?
my reason is simple - range. Ignite and Exhaust have silly range, compared to Xerath's abilities and demand that you get close to opponent, which usually edns bad for you. Most beneficial spells for Xerath are those, who work on him, not his enemies. Also, you slow from Rylai.

Runes
I went with flat AP quints, flat ap glyphs, mp marks and health per lvl seals, to counter squishiness a bit more, and to ad a little punch early on. Viable runes are also mana regeneration instead of health or flat ap all, except marks.

I start off with a Doran's Ring - more hp, more ap, easier to lane. I go for boots from there - Lower Cooldowns mean quicker punishement. Then comes the tear, for sustanied laning, for xerath is mana hungry, if you constantly poke. You can also switch boots with tear, it's up to you.

Then comes some heavier things - Rylai is my first pick, for it helps at landing skills a lot, when you E, hitting Q is like stealing candy from children.
I pick Will before Rabadon for a simple reason - the range of Q means you can hit a whole Wave of Creeps, which means a ****load of healing. It's priceless to look at gangplank, struting around your turret, as he watches helplessly as you get back to 80% hp from 2% with two spells. :)

Rabadon - obvious reason, more punishement.
Then I complete Archangel, as it grants you generous amount of AP. Last slot may be used for some defensive items or Void Staff, if their team is super heavy Magic Ress, but it rarely happens, at least when I play him.

Masteries
I go 9/0/21. 9 is pretty self-explanatory, more AP and Magic Penetration.
In Utility tree, I upgrade my Teleport and get good hands, for is way less usefull. I take experience over expanded mind - when you have tear, it's a waste of points and if you don't - even more. Then I pick mana regeneration, gold and one point in longer neutral buffs. Then it's upgrading Flash and some more movement speed, and cdrs from top tier masteries.

Now for our bread and butter - the super skills of Xerath.
- we get sweet 15% ap bonus to or aurmour. This by no means make Xerath a tank, it's just a nice addo to tons of ap.
- This skill is Xerath's main damage skill and what skill it is! It deals lots of damage, at amazing range, when W is used. This skill is to be maxed first, to get the most of it. It's a perfect poke and you should use it whenever you can, when you have tear, otherwise you need to be a bit more careful, for mana will run out super quick.
- This skill, aside from increasing your range, grants you two things - magic penetrations and a lot of it, and also grants you a speed buff for few secnods, therefore making ghost not a viable choice. Overall, it's a nice buff, but it has drawbacks - at low lvls it has terrible cooldown of 20 secs and it also takes around 0,5 sec to "perpare". Xerath is immobile while using it, but he still can be taunted, feared, etc. It should be maxed secondary, for cdr decrease, so you should be very careful in using it, if you are solo somewhere.
- E is a stun poke and it should be maxed last, since the stun doesn't increase with level, it's always 1.5 This is a good combo start, for either early harrasement with Q or later-on going for the kill with R. It should be maxed last, but picked at lvl 2 for stun effect.
- this is the Orbital Cannon of Awesome. It has a Kog'maw range when in W, can be casted in a sequence of 3 shots, and has silly low cooldown for such powerful ability. At lvl 18, with CDR it's around 30 secs. This skill should be used after E, so that enemy won't run. It should be casted with a bit of insight - try to guess where your opponent will run, and he will, thx to it damage, since you can't blow all three on his head while he is stuned. You should not be afraid to spam it - it has low cd and costs mana only for first cast, so it's a perfect tool for holding off enemies, who are too aggresive. Max it whenever you can.

So then, my guide is concluded. I hope it will be helpful and make you understand this powerful champion a bit more. Share your opinion in comments.