Not Updated For Current Season
This build may seem to be weird for him but it is great. This makes it that they can never get away. With this survivability he can blink in, and take out their weak carrys and then blink out. Only tanks have a chance against Kassadin. He may be played as a tank, but he is also a spell caster. Nether Blade helps him do a lot of damage around 150 to most targets instead of 110. His mana is perfect for him, and never runs out. When it gets low just fight some creeps and back to full.
Starting out with heal makes leaking first blood impossible. When low on health use then they don't realize you have and first blood is yours. The health potions will heal back any damage taken after first sblood. Getting first blood is essential to get ahead. It allows you to get a ruby crystal right a way. Cataclyst becomes a one minute item to get. Don't go back right away though, wait for 1000 gold then teleport back. Buy the Cataclyst and teleport making you loose 40 xp at the most. Now that you have that the real fun begins. You will be level three probably one level ahead of everyone making your null sphere do great damage. Harass as much as possible. Once your mana is gone then the cataclyst will refill it all the way up. After getting level 6 you should be able to riftwalk, null sphere, force pulse to get more kills. Around the 10 minute mark you should have a phage farmed.
Now Frozen Mallet is probably accesable for you now with a couple of kills. During mid game before any hero gets to good with late game is crucial. With beserker greaves in hand you can never let a kill get away. The slow will destroy from the mallet so no one can run. Try to gank your lane as much as possible, and teleporting to lanes with enemies with low life. Even with some one else there you will not die. Try to farm in your lane with the most creep kills.
Guardian angel and lich bane may be a rare item to get. It depends how farmed you are. Guardian angel will prevent the death allowing you to teleport out. Now just push and get targeted to win.