This build is based entirely on ability power. You have to buy mejai's soulstealer in early game. If you do soon get a lead on ability power. You also need anyway Rylai Cristal's Sceptre and Rabandon's. Rylai Cristal's Sceptre is useful because it can slow people so you can easily use Call of the void on them. Rabandon Deathcap's gives you a lot of ability power. These are the items that you should have anyway. I hope you find it useful.