Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is the best build for Kassadin once you can play him at a high level. I would say enough said, but I will explain a little further.
The first item you build onto is Tears. Tears will carry the mana for the rest of the game provided you don't play completely aggressively and shoot out Sphear's on cooldown and mainly just use them to harass in the lane (you should be taking middle).
After that you go straight into a Soul Stealer. The reason Kassadin benefits so much from the stacked items (Soul Stealer and Levi) is because he is difficult to kill due to his teleport. Kassadin doesn't lose his stacks easily because you can simply teleport over the wall and get away clean in most bad situations (play smart and conservative, don't initiate).
You probably realized the skill sequence was not real, and that is because I'm lazy. You should level your Q first and then level E over Q whenever you have the chance. Level your R over anything else. So skill priority for each level (after level one) is: R>E>Q>W. W is essentially worthless and you should only level it if it is the only option left, and only use it to build up stacks to use your E. E does most of your damage, not the Orb. Remember that, it's really important and alot of Kassadins think the Orb is the main skill, and it's not. E does far more damage.
I used a heavy offense build because Kassadin is an offensive character- I am 100% offensive when I play Kassadin (although I do bait and run as if I'm scared of the enemy champion often to score easy kills, it works because Kassadin is squishy).
I have tested all the builds here on Mobafire. This build is the best. Try it, you will be suprised at how smooth things turn out after you get Sorc boots.
A note about Summoner spells: Ignite and Heal are the best I have found. Ignite doesn't need an explanation, but Heal on the other hand should be explained: I use Heal primarily for baiting in the lane. For instance alot of the time I will leave myself at about 25% and the guy in the lane against me will be somewhere around 60-70%. I will B and he will come in closer thinking he is going to score a free kill, and then I pop heal, Teleport on top of him, E him and then Q him, toss Ignite on him and Telport away. Simple, effective, and more stacks for my Levi and SS. I think because of Kassadins insane burst potential that Heal works for him better than most champions since it can be used in this way as opposed to just giving my hero a little bit more health to stay in the lane longer. I've also used it to save my team member's lives countless times.
Dont minus me without feedback, thanks!