Hello Xerath is a Caster Tanky-DPS killer champion. He preforms this role well with his Free Gold Passive, Massive amounts of magic pen, and with good positioning that should be known by YOU the player. Xerath reaches his full potential in organized team play however he can still be used to Pubstomp n00bs, grab those kills when you say "karthus ulty" in all chat and ,the best part, allows your team to hold an advantage over an enemy once you get into the position snowballing himself into an unstoppable siege tank.

These are the runes i use/recommend the runes you decide to use are ENTIRELY your choice and SHOULD be varied to fit in with the team you are part of.

((Greater Quintessence of Potency)) - I typically roll with these Quints as they provide a HUGE power up to your early on damage.

((Greater mark of insight)) - I believe I established that Xerath is a Tanky DPS killer as such he needs to penetrate that tanky part , I belive these fit the bill for that :D.

((Greater seal of clarity)) - Xerath is a champion with mana problems that is a fact BUT his mana problems do not begin until 6+ so I use these seals to counter that so I am not blue reliant you NEVER want to be reliant on blue, if a champion is hard reliant on blue the minute his team loses control of the blue buff things start going bad so trust me when i say haveing these as a backup is a good idea even if they dont provide that much extra.

((Greater glyph of Force)) - I use these runes on xerath since i gain a good starting boost from my quints these can be used as a decent boost all game to your magical prowess.

I run a 9-0-21 , the standard caster run,
I take 9 in offence to get that extra magic pen since I do not max out my locus first to allow myself to better fight those who build typical MR items (IE early merc treads or negatron cloak)
I take 21 in the utility tree for the various useful bonuses it provides for a caster.

On xerath I start out boots 2 healing pots and 1 mana pot (or 2 mana 1 healing) I need the mana pot to make up for my lack of ealy game mana regen from runes, my one pot should last me all the way to 6. I lane until I have at least 950 gold at this point I push my lane and recall for 2 doran's rings allowing me more mana Health ability power and mana regen for laneing. I continue my lane and at this point aseess am I having an average game in which case I B and buy sorc boots at 750 gold or am I having a stronger lane (2 kills ish) in which case I B when I get 1600 gold and buy a needlessly. whichever you decided on pick up the other item next time. From here I buy my deathcap at this point the Pwnage begins. Now go for a rylai's crystal scepter for the bonus AP health And a slow to use for kiteing. Follow up with a moonflair blade for the tenacity Morellos evil tome for Cdr mana regain AND Ap all important factors on xerath then buy a banshees to round out the build with hp mana mr and a spell block.

As xerath max your Q first for damage, harass, and farm
Take a point in locus of power and max it last reasoning behind this : the duration you are in this skill is a flat 8 seconds on all levels the only thing changing is the CD and the M pen with the time you are in it you will only typically need it once a fight, the morn is only useful for high lvl lategame fights when they are buying heavy MR items the movespeed boost is useful but its not useful enough.
I take a point in E at lvl 3 and max it second for the burst the stun availability and the CD
Put a point in ult whenever u can.