PLEASE READ

1) The build order is for an optimal game, IE your dominating your lane. I extremely rarely have this build
2) I don't really know how to hyper link on this for abilities and items, so anyone interested in bettering this guide with those just send me a message.
3) very open to suggestions of bettering this guide, message or comment's appreciated immensely.

Hi, I've been leaching off of MobaFire for a few months now seemed like a good idea to try and assist the community. I've looked over the Xerath builds and find that most of them I find somewhat lacking for early game which if your going to mid is essential.

I'll go through my play styles and some optional builds, some specific champs.

Pro
ridiculous range
low cool downs
hits like a truck
stun
ult can devastate a team fight

Cons
weak to CC, IE fiddlesticks fear
is squishy with this build
stun needs you to hit two spells not optimal for getaway

The rune's I picked are for your early game, by the time you get your boots you have %30 cd reduction (roughly) 15 base magic pen plus %15 not including locus of power bonus, that's enough to get through %90 of early game magic resist, when people get to the point of there build when they can stack mana resist, abysmal specter or void staff (depending how much is stacked) will nullify there purchase and you'll have the bonus or extra AP

Standard AP mage masteries, if it ain't broke don't fix it.

side note, I'll occasionally switch perseverance and good hands because if you have a nice team set up can go all game with very few deaths.

Arcanopulse is your bread and butter, it's your harass, it procs your stun, it's your farm mid game there's no reason not to max it first it does everything for you.

Locus of Power makes your good range ridiculous and has magic pen built in, this is the reason i go with cooldown boots instead of sorc shoes. I've seen a lot of Xerath's not abuse the speed boost it gives, turn it on and off real quick to surprise charge enemies, or if you have a small head start while running away can really widen the gap. the range on arcanopulse without locus is roughly the range of Mage chains with it, so if your not using smart casting you can range check with Q.

Mage Chains surprising damage when leveled, but the stun is what your gonna use it for most, the projectile is kinda slow so if a champion is running away and you cast your arcanopulse, the pulse can hit before the mage chains. if your quick enough at casting with Locus of Power active you can hit mage chains and arcanopulse at almost the same time, which is key to let you get 2 ult pulses off before the stun wears off.

Arcane Barrage - not much to say, it's an amazing ult has a low cool down, can and will change team fights. most people like using this to proc stun with there kill combo, I prefer to use an arcanopulse, followed up with this since you can get 2 off before the stun wears off, and the range is so incredible get the third off while they are running.

Doran's Ring is an amazing starter item for Xerath, ap, health, mana regain there's really no other way to start him imo, some people like to go rush tears starting with Sapphire Crystal but I feel that Xerath should not be spamming spells early and only using spells to harass, if you do that the mana regain from the ring will keep you in lane.

Amplifying Tome helps with Xerath's lack luster starting power and again I feel as long as your mindful of your mana you don't need to get tears yet. If your really owning your lane might jump straight to Mejai's Soulstealer here since getting kills before having it is kinda a win/lose.
Alternatively if your not doing so hot forget mejai's and build into fiendish codex, then get some mana items since your gonna be able to spam that little bit faster and will need to farm with spells.

Boots, most people go Sorcerer's Shoes, but with the built in penetration with W a little extra cooldown reduction from Ionian Boots of Lucidity can help you get more stuns out in team fights.

The rest of the build is dependent on enemy composition and how your game is going, if your running out of mana get tears first, but you can solo blue golem if you get death cap.

Void Staff because by this point of the game you should be feared so people should be building magic resist, if there not building magic resist I've seen a few Xerath use Will of the Ancients for survivability and AP, is worth considering.

Zhonya's Hourglass i just threw this in because in the last game i played i had a Master Yi keep charging through my team to me and try to drop me before the fight got started, popped the active, and my team slaughtered him every time. However this item is entirely situational, if you followed the build to this point you don't need more ap for dmg, I had one game where an AP Teemo had mushroomed everywhere so i got Force of Nature. I suggest something for some survivability since at this point you should be fighting in someone's base.


Options

There are a lot of optional items to consider with Xerath that are applicable in certain situations, if you can't get your combo to finish for instance, Rylai's Crystal Scepter makes it ridiculously easy to do so. The only reason I don't main this item is that, with my play style the extra health is almost a wasted stat since if your getting hit, your out of position.

Other items to consider is Morello's Evil Tome, mana regain cooldown reductions and AP, it's an item worth considering if you insist on getting sorc shoes, as you'll hit the cool down cap stacking both evil tome and boots of lucidity. the best time for this item is if your having a really rough laning phase, and i would replace mejai's with this and get sorc shoes instead of boots of lucidity.
Another popular item is Deathfire Grasp, gives you alot of nice stats but that active is going to roughly hit for %47.5 of there remaining health with the 500AP you can easily hit. (%30 base + (3.5 per 100 = %17.5) = %47.5) however as attractive as that is you need to build a lot more survivability since it has a short range compared to your spells and your going to get hit using this. That said, if there running 2 or more people who stack health this can really make those tanks think twice about just running into your team.
Abyssal Mask is getting some praise for Xerath builds, and looking at the numbers i have to agree, if people have enough magic resist to give you problems but not enough to need void staff (60-90 range) it's a great little item, i passed over it since ideally you don't want to be close enough for it's aura to work, but the aura goes about the same distance as your locused mage chains.

For Mid
Early game - most champions cannot deal with your range, and you have the advantage that your skill shot goes through creeps with no negative effect, so keep creeps between you and your opponent. don't use your spells to farm if you're decent at last hitting it's not needed and saves mana for your harass. Speaking of which, you want to use your abilities as counter harass, when your oppenent starts to come forward, arcanopulse him a lesson, when you have your mage chains anytime you can hit your opponent with it do so but don't chase to do so, your squishy. using your locus of power makes it easier to harass and hit your stun, also makes you hit harder and has a surprise not a lot of people use, trigger it and most opponents get well away, turn it on and off quickly and it's a mini ghost and you can get in a low health champs face faster then he expects.

Mid Game - if you have blue golem and cool down boots, your spells are extremely spammable, 3-4 seconds on your dmg dealers slightly more on your W, your ult when your in locus has the craziest range, deals okay damage on each pulse but hitting all three is how your going to get kills. hiding in the jungle and using your locus range so you can hit people without ever being in danger is something you must abuse, if you are getting targeted you are out of position. your range is abusable, do so, your team mates might get a little upset early game since you have to be extremely cautious in order to get your mejai's stacks up, but with your range you will still be an asset in any team fight. you should have your locus up anytime there's a team fight, the extra range and magic pen is extremely helpful, but if the fight is mobile don't be afraid to end the effect early for your speed boost to escape or pursue. your kill combo is Q, E, Q, R, R, R, Q if anyone is still standing after that you missed I don't consider W as part of the combo but you should have it on preferably the whole time but not many people are gonna stand there and let you wail on them, so at least get the stun and one ult pulse with it on, turn it off and chase if there running or flee if the fight went bad.

Late Game - you should be getting targeted now, but if your in position and abusing your range with W there should be an entire team between you and your targets. Remember an assist is as good as a kill, a lot of carries seem to think getting the kill is more important but it's a team game, if you get 4 assists instead of chasing a single kill your team will be better off. starting fights with your ult is not a bad idea since you can take half there entire teams health if there clustered together. the game plan is stun there carry usually, however there are times when support is a better target or even some tanks, your stun is up ever 4 seconds or there abouts if singed is coming to fling one of your carries, stun him.

there are some match-ups that require some specific skill usage i'll go through a few and if anyone suggests one I'll try to add it

Fiddlesticks - pre level 6 you can play normally but take extra care, if your silenced retreat, since fear and drain hurt a lot and if your silenced you have no way to stop it. post level six, anytime you think his ult is up never cast mage chains, hopefully you can get the stun off while he's channeling ult, but if you can't make sure to stun before your retreat, else he'll flash fear drain and your dead.

Ezreal - pretty easy match up but very important to use creeps as shields, if he can't hit his skill shots you'll out damage him easily.

Caitlyn - again not too hard, since cait's biggest advantage is her range, which she doesn't have on you.

Annie - Counter harass is key, when she comes to hit you with spells use superior range and chase her off.

Katarina - this is a hard match up since she has pretty good base magic resist, hits for mostly magic damage so your passive is useless, and can negate your range with shunpo, abusing the speed burst from locus is important, if she uses shunpo to farm charge as soon as she casts it stun and harass, don't be overly aggressive since her ult will probably kill you outright, always be ready to flash away, if you don't have flash up you need to keep distance and try to out farm with Q.

I hope you've enjoyed how I play Xerath, but this build gets as many assists as kills the way i play it, if you drop all 5 champs to half health to start a fight, that's a lot better then saving your ult to get a kill at the end of the fight. You can also be fairly supportive stunning 2 people in an average length team fight. if you stun a carry or a support there team will have a hard time coping, since you can then drop a ton of damage and your stun combo end game takes half a carry's health.