Hello, I am FoggyPaws, and this is my first guide ever. I wanted to write about Xerath and present my playstyle for testing and review. Focusing on Cooldown Reduction, Mana Regeneration, and Xerath's ability to snipe people from the Fog of War, this build is geared towards being that guy with minions for wards.

Pros

+ EXTREME Range + Poke for days + Harras most others out of lane + Hard to gank from so far back + Drop mortars on Team Fights every 45 seconds + Difficult to run out of mana

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Cons

- Hard to maintain at early levels - When rooted you are vulnerable to gank - Low early mana regen hurts - Easy to gank if on the edge of screen, and... - Usually have to keep Xerath near edge of screen - HP Sustain APC's and Melee APC's counter unless noob

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Nice and easy here.

Greater Seal of Mana Regeneration for early before-item regen.
Greater Quintessence of Experience to get Locus of Power and Arcanopulse maxed out for super spamability.
Greater Glyph of Scaling Ability Power to accent the EXP Quints, making End-Game Xerath even meaner.
Greater Mark of Magic Penetration runes to work with Locus of Power's 40% Bonus!

Exhaust from Early to Mid game is going to be primarily used to slow down a ganking jungler or an opponent in lane who got a little too close and you happen to have Mage Chains and Arcane Barrage available. Mind you you'd use this afterward to keep them around for your small cooldowns.

Flash... Must I explain? Flash is a very good ability to have for almost any champion. You can get out of a dangerous situation by hopping over walls or under a turret, or to turn the tides of a battle by getting into the position you need to secure that kill. Flashing over a wall and hitting them with a swift and lengthy Arcanopulse is a giggle to be had.
Ignite is the only other Summoner Spell I'd say is worth taking on Xerath. Clarity is being removed from Summoners Rift so I'm not using it, but feel free to try it I guess.

This Build focuses on Arcanopulse, Locus of Power, and the Cooldown on them being as low as possible as fast as possible. Locus of Power and Arcanopulse, with the 40%, even 20% from items built in this guide, have very low Cooldown timers, allowing one of the longest range instantaneous Skill Shots in the game become very spammy. That kind of range is hard to counter without help, and in a teamfight scenario, it's not hard to keep your distance and wreak havoc.
Arcanopulse is Xerath's main source of damage, and harass. It doesn't scale as hard as Mage Chains off of AP, but it's cooldown timer is what we are using to our advantage here, and that makes for higher DPS in a fight.

Locus of Power, Xerath's Self-Rooting ability, is the source of Xerath's incredible range and gives all of his abilities more oomph with 40% Bonus Magic Penetration. While rooted with Locus of Power, Arcanopulse has the range to hit someone clear outside the Fog of War. Minions in the lane make for wonderful wards that let you see ahead, and stay out of Enemy View. Mage Chains and Arcane Barrage also have greatly extended range, and benefit from Magic Penetration.

Mage Chains is taken around mid-game, when many times others start to wander from their lanes due to lost towers or because you've managed to get the enemies attention, and need that stun to stop the occasional gank.

Arcane Barrage is used in the build to be used as often as possible. If you have it, use it. Hurt the enemy from a distance. Arcane Barrage also has incredible range when coupled with Locus of Power. Unleash a barrage of energy, make them run home in fear. This often distracts the Jungler or other lanes. You can also use this ability to aid in teamfights, by hiding in bushes, away from the action. This way you can utilize Xerath's range and take out Key Players, like the enemy ADC, APC, or Jungle. You can Spam Arcanopulse in between ult's too, if you want to mix it up!

Athene's Unholy Grail is the first Core Item we go for, as it offers a bit more AP on top of the Doran's Ring and offers the much needed Magic Regeneration and Cooldown Reduction.
Getting this item alone can really give Xerath a ton more combat time in lane or in teamfights.
Morellonomicon is the next item you should get built, as the finisher to the Cooldown Reduction and Mana Regen, as well as add's some extra AP. By now you should also have Arcanopulse and Locus of Power leveled well, and you're good to spam.
I personally say Boots of Swiftness is the way to go with Xerath. Xerath's Locus of Power ability gives a movement speed buff when it wears off or is toggled off. This is a percentage buff, so the best boots allow for dodging skill shots aimed at a rooted Xerath.
That is also why I usually get Enchantment: Alacrity on the boots as well, for a greater buff. Mobility is a huge factor.
Building a Rabadon's Deathcap now would be a smart idea. They're already having a hard time keeping up with the spam (in an ideal situation), so stack up on some AP. Athene's Unholy Grail and Morellonomicon are also beefed up with Rabadon's 30% total AP buff.
If you are doing well enough, you could hold on to Kage's Lucky Pick a little longer and get Rabadon's first.
Rylai's Crystal Scepter will ensure that if you start bringing the pain from a distance you'll be able to land every shot now that the teamfighting is getting heated and more complex this time in the game, if you haven't won by now.

So you've been noticed, Magic Resistance has befallen you. Void Staff, done. Between Greater Mark of Magic Penetration, Arcane Knowledge , and Locus of Power, Xerath is already blasting right through most Magic Resistance. Why not pour salt on the wounds and ignore another 35% of it. (Before Penetration mind you!)

If you've gotten optimal gameplay out of this build you probably want to get Liandry's Torment now. The extra AP and Magic Penetration might not be as impressive as Void Staff, and the HP is nice too, it's the Burning effect this item has that we would be getting it for. With the once every 4 seconds Extended Arcanopulse Xerath basically turns into a long ranged Teemo, applying a nasty Over Time Burn to anyone hit by Xerath's abilities.

You team is made of AP/AD hybrids? Have more AP than AD this game? Will of the Ancients is the way you should go. With Spammy Xerath essentially providing Firing Support for turret offense or behind the scenes teamfight damage. if you're by your team you can give them some Extra Spell Vamp, and AP. The Spell Vamp on Xerath is mean too, considering the speed of your abilities now.

Xerath is a powerful laner, in normal builds.
This is not a normal build. Xerath isn't the most fearsome in lane, until an item or 2 gets built.
Xerath in my build is built to stay back, get as much safe CS as one can get, then start to spam the opponent out of the lane, and use the spamability to regain lost CS, keeping them away from it all.

Getting Athene's Unholy Grail and Morellonomicon are essential to this build, giving Xerath the spammy pokey annoying feel by levels 7-10.
When in early lane, make a presence while you can, line up minions and the enemy Champion and Arcanopulse them early on. Get them to burn potions, but don't miss out on too much CS if you can help it.
Recall when they recall, and always buy 1 or 2 mana potions when you go home.
If your jungler is kind enough to give you Crest of the Ancient Golem then you are pretty much a spam monster from then on out, kick them out of lane, farm up, and build.
Remember to purchase the occasional Ward, don't get ganked.

When the team fighting begins, as Xerath you want to stay at the back of the group. Don't put yourself in a load of danger when you can do just as much damage at a distance. The way we've been building Xerath is for the teamfights. The speed you can spam Arcanopulse and Locus of Power is intense, sending a bolt of energy at everyone but the tank, is going to hurt. You can do this from the Fog of War even, using minions, or Allied Champion vision, or wards. Take advantage of this, stay hidden and follow the teamfighting from the jungle or from lane, depending on where the fight is.
Here's an Example!