This build is geared towards high mobility and damage because of items Cooldown Reduction and Lich Bane.

If you are against physical damage champion, for example Yasou or Talon, replace ignite by exhaustion. The same goes for champions that cause damage per second, as Kayle and Azir

Your 1st round the base is essential to your lane phase, if U are dominating your route and have enough gold, rather than starting with the Catalyst the Protector, you can buy Blasting Wand and The Dark Seal. Rod of Aegis and Lich Bane are essential items for your build their damage and survival capacity mainly comes from them. Because of its Ultimate ability to consume large amounts of mana items like Athene's Unholy Grall is a good choice due to its passive restore 30% of your maximum mana for slaughter or assists If you are against a very Tank team, I recommend changing the Athene's Unholy Grall by Seraph's Embrace and Mejai's Soulstealer by Void Staf

During phase routes bother your enemy using your Null Sphere skill, this is very important because it will cause your enemy to play less aggressive against you, since before 6 lv its damage is much reduced. If playing against a physical damage champion, you must maximize your Force Pulse ability. The Kassadin combo is as follows: When you have six loads of their ability Force Pulse, use your ultimate to cause damage or simply approaching, use Force Pulse followed by Null Sphere, ending with a Nether Blade. If you're lucky your enemy will use some skills providing you with loads more to be able to again cast Force Pulse ability.