HEY IT'S NOT A JUNGLE BUILD FROM ME FOR ONCE. So clearly i have no clue what i'm talking about. If you assume that, you're wrong. I have found a way to throw the ****s you give away and focus whoever you want to in any team fight or 1v1 or small skirmish. Whatever it is, if you get endgame, they will not have magic resist around you. Look that Galio and Rammus through your super awesome sniper scope you have and flip them the bird as you hit them as hard as you hit their carries. I introduce to you all, what i feel is the best way to build Xerath. Read on, and enjoy the science.

PROS

Tanky against AD
Super AoE machine
Super long range
POWER

CONS

Can get tricky to land skill shots
Have to stay in place for super range

ascended formAscended Form
Yah, there are better passives than this. But usually the only champs on the other team that can touch are in fact, ranged AD carries. So this extra armor is actually extremely helpful.

Arcanopulse
Your super awesome i feel super sweet AoE nuke ability. High damage early on, as well late game. It's basically a railgun. It shoots electricity from a far distance at your enemy's face. What's not to like?

locus of powerLocus of Power
This is what makes Xerath the champ that he is. This is what makes you feel awesome. This is your sniping ability. Out-poke an Ezreal and Nidalee all day long. Snipe baron/dragon away from them. Harass them at their turret from the exact center of the map. Hit them from off their screen and make them "wtf?" It's super awesome, and makes you feel super awesome. But the most important thing about this ability to remember is not it's range, but the 35% spell penetration it gives at max rank.

mage chainsMage Chains
I like this ability personally. It isn't a stun itself, which is good. Ever since Sion was abused and realize auto-target stuns are ******ed they put skill shot or 2-ability stuns into the game. What this means to you. Open with some damage from mage chains and hit them with your Arcanopulse or Arcane Barrage. Don't worry about using your ult on 1 person. If you get the stun off with either of the abilities, the full combo from all of your abilities at level 6 should kill your opponent.

Arcane Barrage
More lightning bolts. Except these ones fall from the sky on their heads. You can use your ultimate to clear minion waves if you want to. It's on a pretty low cooldown and works pretty well from clearing minion waves/full teams. Take your pick. It does a lot of damage over a long range and drops dooks on your enemies.

PRIORITY

Sometimes the priorities can change for which ability to rank up first as Xerath. But the only decision you have to make is between mage chains or locus of power. You always want your Arcane Barrage whenever you can get it, and you want to max Arcanopulse first. But if you're doing well or not feeding mid take mage chains at level 2 and max after Arcanopulse. If you're having troubles you want to get locus of power after Arcanopulse so you can keep your distance and harass them where they can't touch you.

Doing good
-> -> mage chains -> locus of power

Not doing so good
-> -> locus of power -> mage chains

MARKS

Greater Mark of Magic Penetration - All AP casters have it. However, endgame Xerath is going to benefit from them more than most other casters.

SEALS

Greater Seal of Scaling Ability Power - Not the most powerful level 1, but will be level 6.

GLYPHS

Greater Glyph of Scaling Ability Power - Same idea, you're going to want to have some real power and be real terror at level 6.

QUINTS

Greater Quintessence of Ability Power - Why not per level anymore? Because, this gives you a foundation for the seals and glyphs to build off of. With all per level you'd have nothing until level 6 and be a pansy in your lane.

Important Mastery Points

Arcane Knowledge
To be honest, this is the only mastery point in all of the possible points that i feel is important to Xerath. You can go 9/0/21 or 9/21/0 or 9/?/?. It doesn't matter as long as you have the 10% magic penetration.

Good Summoner Spells for Xerath

Flash - Use it to escape over a wall and snipe them before they can try to catch up.
Ignite - Make sure you get that first blood.
Cleanse - Now gets rid of ignites. IMO, stupid. But can really make people rage.
Exhaust - More magic resist shred. But as Xerath you should not be getting close enough to even put Exhaust onto someone


Decent Summoner Spells

Ghost - You shouldn't have problems closing distance since you are a sniper.
Teleport - Sure, can be helpful to push lanes. But never ever want to use it to TP into team fights.
Clairvoyance - Use it the way Lux does to snipe people. But not the best.


Trolling Summoner Spells

Clarity - I've only gone OOM once as Xerath. You don't need it, and you don't need mana regen as long as you aren't spamming your abilities to farm.
Revive - LOLYUDIE BADKEED
Promote - LOL no.
Surge - Because you're attack speed based, right bro?

and Boots and Health Pots
Boots to dodge your lane match up's skill shots and health pots if you still don't dodge them or exchange.

Doran's Ring
I'll usually buy 2-3 of these for early game. It helps, quite a big deal to just grab these and run into the lane with more health, power and mana regen. The only way you don't get this the first time back is if you can flat out buy a Needlessly Large Rod. If you have that gold your first time back, then by all means, get the NLR. But these will help everything go smoother, especially if you're laning against someone that doesn't have mana like Katarina and you need to stay in the lane as long as possible.

Sorcerer's Shoes
20 points off of their magic resist when you hit them. Lovely, and much better than any other boots for Xerath.

Rabadon's Deathcap
You are an AP caster, if you don't get this item as an AP carry, you're building them wrong and won't do nearly as damage as you should. So rush this after 2-3 Doran's Ring.

Rylai's Crystal Scepter
All the health and ability power you are going to need for the rest of the game. After you can have built Rabadon's Deathcap and can build the Giant's Belt from this, you can sell those Doran's Ring you've been hanging onto to speed the process up.

Void Staff
Ability power. Nice. 40% Spell penetration. More than nice. Get it.

Abyssal Scepter
It gives you and your team magic resist. It debuffs their team's magic resist. Is it a tank item? Sure. Is it less expensive than a Banshee's Veil? Yes. Is it better than Banshee's Veil? For Xerath, it is amazing. You now are rather tanky with your passive giving you tons of armor and now have some decent magic resist.

The Last Item Choices
I assure you Zhonya's Hourglass is not a good option. Granite that yes, you will have over 200 armor with this but their team shouldn't be coming close to you to begin with to even have to use the active. I put Will of the Ancients up there for a reason. Because if you have lost any damage you can quickly move out of range of their attacks and snipe them back for the damage they dealt leaving you, ahead. However, i do also feel that for durability's sake and more ability power that a Lich Bane could also be a pretty good item for him, despite usually being out of auto-attack range.

Alright kids, time for some math. Listen up, and pay attention because this is why Xerath can deal more damage to your tanks than any other caster can with this build. Let's start with the percentages.

locus of power - At max level, 35% magic penetration.
Void Staff - 40% magic penetration.
Arcane Knowledge - 10% magic penetration.
PERCENTAGE TOTAL - 85% magic penetration.
Sorcerer's Shoes - 20 magic penetration.
Abyssal Mask - 20 magic penetration.
Greater Mark of Magic Penetration - 8.55 magic penetration.
POINTS TOTAL - 48.55 magic penetration.
GRAND TOTAL - 85% + 48.55 magic penetration = WTF IS MAGIC RESIST

As far as laning goes, you should always focus on last hitting more than harassing. In some exchanges, especially against other casters, you are not very durable and may only have 1 or 2 abilities that can put out damage. Be cautious when trading. But do know that you will have a range advantage of essentially every other caster in the game. Expect for LeBlanc. She can Distortion in and sigil of silence you and then Distortion back before you can even land mage chains on her. Luckily, she falls off late game, so just be extremely cautious against her and show her what real damage is end game. Also i should point out that you probably won't be able to clear minion waves with abilities and shouldn't try to until you have 2-3 Doran's Ring unless you're behind, then you will need a Needlessly Large Rod to clear the caster minions.

Your job as Xerath is to let your front line tanks and bruisers disrupt their team while you stand off in some far distant brush sniping them for all their health. Remember that mage chains doesn't have a very long range even while in locus of power so put this dot on somebody and create some distance then smack them in the face with some lightning bolts. It is not your job to engage, it is your job to get behind walls or trees and be in locus of power and constantly poke at them with Arcanopulse. But don't be an idiot and always do it from the same spot over and over, because you will get poked back. If you see their whole team grouped up it's time to use your ultimate. Arcane Barrage does a lot of damage with just one use to one person. Imagine the full three blasts hitting all their teammates. Yah, it does quite a lot.

Here are some games i played. Winning.
Here is a game i got to full build and trololol'ed.

I've enjoyed sniping in all video games. It makes me feel accomplished to pull of a shot that just makes people rage or you can go "DID YOU SEE THAT?!" Xerath is a strong caster throughout all parts of the game and can really tear teams apart. He's overlooked and underplayed, and he should be played more.