Xerath is one of the best supports in the game and can easily carry a game.
I'm one of the first ever if not the first to play Xerath support, after doing it for a few times i started to see a few trying it but they weren't as good so i thought of making my guide.

Pros

+ Great poke range early game
+ Great damage late Game
+ can keep your adc safe and farming in peace unlike your enemy adc
+ no need for good adc, you can hold lane alone
+ amazing sustain, you'll be in that lane forever (good damage early game not much in need of items)
+ Shocking Orb has stun, good range and up to 2 seconds
+ abilities goes through minions and champions
+ Rite of the Arcane has great range, you can kill enemies who run away and help teammates who are far away.
+ Scales well with AP
+ Almost never banned and enemy won't expect it

Cons

- Vulnerable to ganks (for beginners at least)
- Shocking Orb stun duration depends on travel, if enemy like Shaco gets too close you'll probably die.
- some adcs won't like it because your awe abilities can take farm so be careful.
- has good sustain but mana hungry early game
- abilities a bit hard to hit for beginners (but not hard to learn).

1.One of the most important Xerath potential that no other guide seems to mention or notice is sustain, staying in lane as much as possible, a lot may not know this but Xerath has one of the best if not the best sustain capabilities in the game. his passive Mana Surge keeps your mana up and Taste of Blood keeps your hp up in case of enemy harassment.
2. Xerath main strategy is keep harassing the enemy champions with your Eye of Destruction and Arcanopulse. it can be real annoying and you keep them on low health.
3. you have the range advantage, keep your distance, play safe, your Shocking Orb stuns for longer the more it travels.
4. Use Shocking Orb at a distance in case of ganks or enemy attacks.
5. Vision, you need a lot of it, for keeping an eye on potential enemy ganks and (really important) seeing where the enemy runs when using your Rite of the Arcane
6. your Eye of Destruction has slow and 100% AP when hitting the middle, use that, when being chased or when enemy is stunned
7. your Arcanopulse needs charging, enemies can see that, stay in bushes they'll have no chance to dodge.
8. your Rite of the Arcane can make you a long range support, if one of your teammates is having trouble (2v1 or 3v1 or against good enemy) use your Rite of the Arcane to barrage the (probably non-moving) enemies giving your teammates a huge advantage and probably saving them.
note: Don't wait till enemies get to low health so you can take the kill helping teammates comes before kills they might die before you use it.
9. very important: Attack enemies with Eye of Destruction first, its slow makes sure that Arcane Comet almost always hits.

My Xerath build is based on fear, the enemy needs to fear you and fear getting hit by your abilities, that way they keep their distance which is what you want.
for that to happen you need a few tips:
1. Abilities need to hit like a truck, go magic penetration and AP.
2. if enemy tries to get close punish them with Shocking Orb then Eye of Destruction(hit the middle) and Arcanopulse. if they run and you have time you can try to finish with Rite of the Arcane.
3. harass non stop, whenever an enemy gets in range hit them.

Arcane Comet- you're poke focus so it's the best one, Summon Aery is ok but you give no shields so it's a waste.
Manaflow Band- you may have good sustain but you're mana hungry, your passive won't be enough when you're spamming those abilities late game.
Transcendence- is my go to rune because of the cooldown on abilities (especially your R Rite of the Arcane), Absolute Focus is also good.
Gathering Storm- is one of the most important ones, as i said Xerath scales so well with AP, anything else is just a waste. some put Scorch on their build which is a real waste because your abilities scale so well with AP, Scorch is used for extending burning from the likes of Demonic Embrace and Liandry's Torment.
Taste of Blood- is the most important in this build, its healing from pokes gives you much sustain in laning phase you'll be there forever.
Ultimate Hunter- lets you use your Rite of the Arcane more which will secure kills more than you expect and helping teammates.