This build is focused on giving Xerath an insane DPS with an extremely high AP and almost max cooldown reduction, to be able to spam spells and deal insane damage. At level 18, with all listed items, you will have an AP of 739, and with your 39% CDR you will be able to fire your Locus of Power + Arcanopulse every 3 seconds. And with Xerath, you ALWAYS have a target, due to the insane range.

The build will also give you an awesome mana regeneration that - despite your spellcasting frequency - will make sure you NEVER run out of mana. As a bonus, this build also eliminates some squishyness, with health bonus from Rod of Ages, armor bonus from Ascended Form and MR bonus from Lich Bane. The build works best on mid-lane 1v1, and in my opinion, Xerath is the best mid-laner of the game.

...with Xerath

+ Overall longest range of the game
+ A lot of AoE
+ Excellent farmer
+ Excellent carry
+ Good at ganks, if needed for that
+ Really cool voice and cool look

- Not the highest default DPS compared to other AP's like Annie or Brand
- Easy to fumble with (ie getting stuck in Locus of Power)
- Hard to master for optimal damage
- Pretty chanceless in 2v1 (ie early-game ganks)
- Needs cooler skins

...with this build

+ Very high AP
+ Very high mana and regen
+ Practically maximum CDR
+ A little anti-squish
+ Can out-range anything at all times after lvl 12

- Doesn't get around very quickly (ie no ghost or teleport)
- Umm... I really don't know what more to say!

The runeset for this champion is pretty standard. Marks of Magic Penetration because that is the only primary mark that works for AP offense. Seals of Mana Regen because this build is suffering from low mana regen (flat ones, since the mana regen won't be a problem after you have Morello's Evil Tome and Archangel's Staff), Glyphs and Quintessences of Ability Power because we want all the AP we can get.

You might wonder, why level-based AP glyphs, but not level-based AP quints? Simple: One Greater Glyph of Ability Power gives more AP than one Greater Glyph of Scaling Ability Power between levels 1-5. But at level 6 and up, the level-based ones are better. But with Greater Quintessence of Ability Power vs Greater Quintessence of Scaling Ability Power, the ratio is higher, so the level-based ones will not be giving more AP until level 12.

Since the new mastery trees, aside from the utility tree, there is now also an offense branch that is very beneficial for a carry mage. This branch focuses on damage and ability power, but will also grant you an extra 4% CDR and 10% magic penetration. If you choose the AP branch on the offense tree, there really aren't many viable options aside from this one. I'd say the only options in this build is the utility tree, where you may get Runic Affinity instead of Transmutation , that is of course if you find yourself using monster buffs a lot.

With the 4% extra CDR, this build will give you 39% in total (40% is the maximum the game allows you to have), which is basically as good and efficient as it gets. It will also grant you another 10% magic penetration, leaving you at 40% in total when using your Locus of Power. However, if you choose the Void Staff instead of Lich Bane, you will end up with a total of 80% magic penetration, which will satisfy any need of magic penetration one might have. There are four different kinds of magic penetration: Percentage magic resist reduction, flat magic resist reduction, flat magic penetration and percentage magic penetration (applied in the mentioned order). Xerath is a hero that bases a lot of his magic penetration in the percentage magic penetration field, and if you add a lot of flat magic penetration to that, you will actually decrease the efficiency of the percentage magic penetration. Therefore, this build is extremely beneficial in this sense.

Due to the low cooldown this build will grant you, you you WILL be able to waste all your mana if you try hard, especially early-game. I find that Clarity helps me enough early-game and mid-game to ALWAYS be able to keep my opponent at a distance by using Arcanopulse every 3-4 seconds. I fire my Arcanopulse constantly. If not on my opponent, on minions. Selecting Clarity will - worst case scenario - at least grant you LOADS of gold from farming.

Flash is still the best getaway spell, despite the recent nerf.

Here is a list of other summoner spells (in order of relevance), and why I don't pick them:

Clairvoyance - As the main AP carry, I usually rely on having my jungler ward well. Hence, enabling more damage dealing is a better option.

Ignite - Good spell for this hero, but the range of it is just too short. Since this hero relies on keeping the distance, you rarely get to use this efficiently.

Exhaust - Same as above. You might use it to slow down enemies that chase you, but for chasing, you're rarely close enough to the enemy hero anyway, and if you are, you're much likely able to reach him with your spells to finish him off anyway.

Ghost - If played properly, you sort of have a built-in ghost ability with this champion, when exiting your Locus of Power. While this spell is a good backup if the enemy is intelligent enough to target you, I prefer always being able to deal constant damage, hence not allowing them to come close to you in the first place.

Heal - The main goal is to not put yourself in a situation where you will need this. But if you're targeted a lot, and if you like the spell...

Teleport - I find that for this spell to do one justice, one should be able to perform stunts with it, such as teleporting to and finishing off an enemy tower that your minions have gotten close to, or tele-ganking a lone enemy hero that is farming. But AP heroes suck against towers, and this hero is pretty useless in 1v1 (if it isn't laning).

Other spells are just not even up for discussion...

Xerath's passive, Ascended Form, is what gives him potential to be not-so-squishy. A high AP boost, which is required for this hero, will grant him armor, and adding just a little health to that, he will not be the most optimal target for an AD enemy.

This chapter is a no-brainer. Start with maxing out Arcanopulse (Q) since it's his main attack (longest range, highest damage, lowest cooldown, multiple targets), and add Locus of Power (W) after that. The main reason to level your W is for the cooldown reduction (at level 5, the CD is equal to the CD of Q, which gives you constant long range, constant extra magic penetration, and constant speed boost when you're not firing). Last but not least, Mage Chains (E), which is mainly used for stuns early-game.

Also, would like to point out that the ulti, Arcane Barrage (R), is one of the best ultis out there, if used properly. When the enemies are clumped up and pushing (or trying to gank one of your team mates), this spell will do tons of damage. Try to hit 2 or more champions with each blast at all times (except for when you're using it to finish someone off).

First item to get immediately is Meki's Pendant, for the mana regeneration. You upgrade that to Morello's Evil Tome in parallell to buying your Ionian Boots of Lucidity. These - combined with Clarity - will give you a good damage output at all times early-game.

The next choice for me would be Archangel's Staff, which you need early because of the mana stack (it requires you to cast 250 spells with 3 seconds cooldown to reach maximum mana). By the time you get it, your mana will probably be around 1500, so you will receive a 45 AP bonus on top of the 45 AP in the item, so it's still a good AP boost.

The order of the next items sort of depend. If you find yourself being targeted a lot, I'd start with Rod of Ages because of the health boost. If the enemies targeting you are mainly AP heroes, I might consider getting Lich Bane after. But if you're doing fine, having no problems at all, I'd go for higher damage output and buy Rabadon's Deathcap first.

If the enemies tend to bunker up on MR (that might happen if you play Xerath right), I would consider switching my Lich Bane to a Void Staff. That, together with Locus of Power and Arcane Knowledge will give you 80% Magic Penetration, which will pretty much render all MR items useless to you.

As mentioned, this build is optimized for mid-laning. Early-game, this hero is awesome (probably the best) at harassing the enemy with Arcanopulse (Q). The low cooldown allows you to fire a lot, and you can always stand behind your own and the enemy minions while firing for simultaneous protection and farming. Locus of Power won't be that good in the beginning, since the cooldown is sort of long, but can be used every once in a while for a "secure shot" (ie a shot without having to take the risk of being fired upon).

Regarding Locus of Power (W), there are a few things to have in mind:
While in the lane together with your opponent, never remain in the immobile state for long. Just use it to fire the long shot(s), then deactivate it, otherwise you're making the skill shots so much easier for the opponent. However, when your opponent is missing/dead/healing, and you're harassing the opponent's tower, go to the immobile state and remain standing like that for as long as possible. That way, if someone tries to gank you, or the enemy team's mid-laner gets back, you can toggle it off and use the speed boost to flee back.

With this build, I claim that in a 1v1 lane, you will always have an advantage against your opponent, due to the range. The only way to beat this is if the enemy team has a jungler that ganks a lot. But even then, this hero is in a good spot because of the distance he is always able to keep.

In later game, the best sequence of attack is:

Locus of Power + Mage Chains + Arcanopulse (stun) + Arcane Barrage + Arcane Barrage + Arcane Barrage + Arcanopulse
W + E + Q + R + R + R + Q

You will have time to do this in one single LoP. When pushing, I recommend keeping your distance and just repeating Locus of Power (W) + Arcanopulse (Q) as often as you can (every 3rd second at lvl 13 with this build) until you get into a big gang fight. That's when you choose your target (that is in the middle of the enemy ball) and fire away.

NEVER GO OFF ALONE! This hero is useless on his own in late-game, so make sure you stay with your team (or at least your tank) at all times.

I've done so INCREDIBLY well with this build, and I have NEVER lost the midlane war. Everyone should try it!!!

I know this guide contains some non-standard things (like Clarity) that many might frown upon. All I can say is: Try this build before you judge it!!! You WILL have good use for it!

That's all folks! =D