I get the Arcanopulse first to help with farm followed by Mage Chains to keep aggressive champions back with stun. I grab Locus of Power third to extend the range of my Arcanopulse and increase my farm and raise potential for ganking. I switch between Arcanopulse and Locus of Power to get the advantages of the damage as well as the magic pen from Locus of Power quickly bringing in Arcane Barage whenever it becomes available for the full burst damage potential.

I start of with the Mekai Pendant because frequent use of the Arcanopulse will easily lead to an oom situation. You can also grab Doran's Ring if you're more comfortable using the Doran's items to start with.

I follow up with the Fiendish Codex to get the extra CD and if I'm having problems harassing I'll grab the Boots of Speed after to help me move in and out. If I'm not having any trouble with movement I'll upgrade to the Morello's Evil Tome first. After I've grabbed the Morello's I'll grab the Ionian boots so that I can increase my harass with my Arcanopulse a bit more.

The third core item I grab is the Haunting Guise because it is a cheap way to bring in that little extra bit of magic penetration, this can easily be switched for Void Staff if early Magic Penetration is required. I follow this up with Tears of the Goddess because this much harass starts to drain mana much quicker and you need to start building the mana into the item by using abilities. I upgrade this to the Archangel's staff as soon as possible.

The fifth core item I grab is the Rabaddon's Deathcap which at this point adds a significant amount of AP.

The sixth item is entirely dependent on the enemy team makeup. I like to grab the Zhonya's hourglass because it has the additional AP on it but if the enemy team has a lot of CC I may go with the Banshee's Veil instead so that I'm not prevented from dropping some heavy damage right way.

The last thing I'll do if I manage to get enough gold is to replace the Haunting Guise with the Void Staff just to secure the last bit of Magic Penetration.

If the game progresses to this point I'll grab the Elixir of Brilliance.

Since Xerath dose tend to mid it is useful to purchase Wards throughout the game and Oracle's Elixir if there are enemies on the team that are using invisibility heavily such as Akali or Shaco.