Hello there, my name is ShrO0M I'm just ranked in silver but I'm playing League mainly as support since 2011, so I had a lot of time to get some knowledge and expierence which should proove the viability of this guide at least a bit :D
It's my first try to make a guide at moba so please don't be too rude and harsh with me.
I realy do like to play unconventional supports like Ashe, Fiddlesticks, Gragas, and so on. I even played Nautilus, Zyra and Elise straight as support when they were released seeing their potential in burst, CC, peel, ability to carry hard even as a support and the possibility to use almost every kind of build; tanky, AP, supportive.....

Pros:

• strong burst, all his abilities do massive dmg
• harass, high zoning potential
• strong cc, Eye of Destruction and Shocking Orb
• sentinel with his Ult Rite of the Arcane

Cons:

• squishy
• no real escape(eg a dash, Dark Passage to rescue dudes)
• no heal
• no buffs

I do recommend to get mostly AP runes:
6-9x Greater Glyph of Ability Power, 9x Greater Mark of Magic Penetration, 3x Greater Quintessence of Ability Power to fit the aggressive playstyle of xerath. Only take 9x Greater Seal of Armor and maybe 3x Greater Glyph of Magic Resist for some resistance in lane. Though Your main goal should always be to burst enemys below half of theitr HP and CC to prepare and/or take kills to carry your team.
3x
9x
9x
6-9x
0-3x

Flash....always take it, I'm sure i dont' have to explain and it's a standard spell but you are really squishy and may be focused because you are one of your teams high burst damage outputs so keep that in mind as well you may need to position yourself to catch up or prepare an initiation/catch/teamfight with Flash+ Shocking Orb+ Eye of Destruction+ Ignite/ Exhaust

Ignite is really usefull due too your early long ranged damage to aim for a fast and early pre 6 kill, getting your ADC or yourself to start snowballing. But in some situations i would rather recommend Exhaust for example if the enemey team do rely on AD and Attackspeed or contain 1-2 Assasins that can jump at your backline.

So far i haven't tested any other spells because I don't see a real use in them accept for Teleport which can be usefull in surprisingly well working tp-ganks, but therefore you need good communication.

Either start with Doran's Ring or with Spellthief's Edge to get even some more exploit profits of your pasive Mana Surge, take some Health Potions and get the Stealth Ward.
Then aim for Sightstone, swapp Stealth Ward with Sweeping Lens buy a Vision Ward for Drake if your time has come and get early on some Magic Penetration either Void Staff and/or Haunting Guise/ Liandry's Torment because if your enemys are smart enough to detect you as the danger source numero uno in bottom lane and/or your Team is mostly AP damage they are going to build Magic Resist as soon as possible, to counter this I always try to get Magic Penetration first as well your enemeys will melt down even faster.



Now you have to decide if your team and yourself do benefit of you being 2nd AP Caster or being "AP-damage-peel-distraction-cc-support". Therefore you always have to follow the game's process , the enemey's and your team's composition....eg is there a Tank, enough damage(fell your AP caster behind), do you need to peel hardcore mode for your ADC.

In most cases you are able to keep on building AP Void Staff, Rabadon's Deathcap, Rylai's Crystal Scepter to burst, poke, seek and destroy your enemeys which will be hopping around like scared rabbits not knowing where to go.

Here are some indicator's which tell you to go for "AP-damage-peel-distraction-cc-support":

• Your ADC is kind of bad/playing really unsafe
• The enemey Team has a lot of hard engage eg [Amumu], [Leona], [Sivir], [Malphite]
• The Enemey Team has a fed/well playing assasin(s) eg [Kha'Zix], [Zed], [Aatrox]
• Your Team has enough damage but get's caught and/or one of the points above applies

You unfortunately had to decide to go this way? So what now...?I will tell you a some possible solutions and items that can help. When you realise your team can't catch up to the enemey or some speed would have helped in certain situations get Talisman of Ascension and or get Twin Shadows it will slow and reveal two enemeys and gives you some Magic Resistance + some movement speed.
If you are really desperate and facing loads of AOE damage go for Aegis of the Legion.

• I do max Eye of Destruction first because of it's AP ratio and it's enormous slow 10-80% for 2.5 seconds
• Arcanopulse 2nd to get some more poke/ long ranged harass
• Alwas take the 2nd point in Shocking Orb to get your stun for initiation and escaping and max it 3rd
• Always skill Rite of the Arcane if possible, it's Xerath's finishing and hard Poke skill being able to help your team even when your not directly next to them.

All in all Xerath is not a standard tanky or pure supportive Support but his ability to do massive damage and CC your enemeys make worth it playing him in botlane. Finally it's your choice if you play him or not to make your own opinion.
Btw I'm German so please be indulgent with my English skills and help me out if I'm missing something :D