hello everyone this is my first guide for league of legends. your going to have to forgive my spelling and grammar i have a small disability and spelling and grammar are my weakest points by far so again please forgive me.
it seems that no one has made a guide about building CDR on xerath and i feel as though people are missing out. so here is what i found works out best for me. if you have any comments, complaints or suggestions please voice them.

pros
great sustain damage
AOE sniping
shines through in tight jungle spaces as long as you have wards
easy farming mid and late game
has a stun

cons
passive dose not help much
very squishy
can be vulnerable to skill shots or gap closers if you are not careful

i know some of you may be ragging at me right now and asking why im going 9/0/21, let me explain with this build you want your cool down to be almost maxed out as quick as possible why'll spending as little gold as possible so you can buy the items that will give you the damage you need. you will end up with 39% CDR with this set up.

so you want to start off with a flask and a faerie charm simply because you are going to get harassed and will have to poke and or use spells occasionally to get cs. with xerath its a bit of a tough start every game (at least for me) but as long as you practice your cs-ing skills you should keep up with your counterpart and poke them down for a gank or a few from your jungle. if they are keeping healthy with pots your doing your job well.
as for your first back try to get a chalice and ter 1 boots in one go. if you can do that you will be allowed to spam your q more often and don't require to take your jungles blue (it seems more junglers require it recently)
as for the rest of the build it seems pretty explanatory but ill go into a bit of detail any way. i get the kage's for the gold per 10 (if you wish you can leave it our or get it and at some point in time build it into a twin shadows so you can use the active to find escaping enemies to snipe and it also makes it easy to hit them with the slow)
finish off the chalice into a athene's for the ability power and the CDR. boots of lucidity for the CDR as well (side note. i was told that the magic pen boots don't work with the magic pen from liandry's torment. im not sure this is true)
the next 2 items can be swiched in the order that they are bought liandry's torment and void staff. id suggest if you feel like your in danger when a team fight erupts or someone has a big gap closer to get liandry's torment first. and finaly for the extra ap so the magic pen is not wasted on small little numbers a deathcap.

the ability power glyphs (blue) for early game damage. your more than likely going to get bullied in lane early game and will have to use your spells to get creeps at a safe distance. you can also get ap per lvl if you would like or if you would like to replace them with magic pen you can do that as well

magic penetration marks (red) just to add more penetration to your spells.

and the gold per 10 seals and quints (yellows and... purple?) this is to get you itemized faster so you can do the insane total amount of sustain damage continuously mid to late game when it starts to get to team fights.

so some of you may be wondering hey why don't i pick ignite instead of teleport. well you could but the range of ignite is about as short as mage chains when out of your locus of power. if your using mage chains when your out of locus of power that is what i like to call an OHH S**T moment. so in all likelihood you would not be using it.
using your ult on minions is not something to be ashamed of use it because it has a short cool down and even if you only have to use 1 or 2 out of the 3 blasts your have a good amount of time before you need to cast the other blasts. no worry's about it delaying your cool down on your ult also. after the first blast the cool down time will start ticking down in the background but you will not see it until you have finished casting the other 2
as far as casting your spells for a full combo. hit W,E(if in range),R,Q,R,R,Q if you are out of range of your E after casting W i would suggest casting Q,R,R,R,Q. why would you do that you may ask? well after you increase your range using W you have the option of casting an AOE line skill shot. or a AOE circle blast. both of which if have the same range indicators so if you aim the AOE circle blast center at the very edge of your range indicator you will in fact have more range than the AOE skill line shot. i know this seems like a small difference but i cant count the amount of kills i have picked up because i have hit my Q before my R.

if your a fan of using smart cast i would suggest putting on smart cast range indicators. even if you don't like to use them on other champions i can almost guarantee that you will love using them on xerath.

if you can help it always try to keep the enemies at the very edge or close to the edge of your spells when using your W

your ult reveals a small area of where it was cast (yes all 3 of the casts). your q dose not reveal anything.

this last one is what is going to probably be the reason for the majority of your deaths if you have any. because mid and late game if you are playing the play style that i suggested then you should not get hit hardly at all so you wont have anything to gauge how much punishment you can and can not take. this is a crustal part of what i was suggesting in the spells section of my guide with using your E (aka mage chains). this is that OHH S**T moment i was talking about and it will happen as some point in time make no mistake about that. believe it or not when your retreating some one will be hot on your tail and you will have a decision to make. do you keep running or do you turn around and tag them with your mage chains and fallow up with a spell to help your team get away. this decision can also be a game breaking one as well. i can not tell you exactly when i have no tricks of telling when to or when not to. i am still struggling with getting good with making the right decision when it comes up but it is so hard when you have almost no way to gauge how much damage they will do to you.