(THIS BUILD DOES NOT assure victory, because that is highly dependent on your team, but if you are a skilled player and follow this build, you will dominate).Kassadin is a fairly difficult champion to play being that he is a melee mage and is squishy as fresh poop, but he can deal out alot of damage in a very very short period of time. This champion requires kiting in order to survive most situations(moving back and forth out of the fight) to be effective. It may take some time to learn to play him if you don't play him often, so i'll go over a few things about him in this guide to hopefully lessen your workload.

Pros
Skill allows flash every few seconds Riftwalk
Silence Nuke Null Sphere
Multi Target slow Force Pulse
Low Cooldowns

Cons
Squishy Melee
Mana Dependant

Runes are just like my other builds, using flat ability power Greater Seal of Ability Power, Greater Glyph of Ability Power, Greater Quintessence of Ability Power and magic penetration Greater Mark of Magic Penetration for early game domination. This aids tremendously in early game kills, so harassing with Null Sphere is strongly recommended. Aggressive play is the way to go, but know your limits. I don't get ability power per lvl runes because i dont feel that they are worth it in the long run, and i am IN THE PROCESS of trying out mana regen runes. I'll update if i decide to change anything, although i am sure that mana regen runes would work great on kassadin.

Masteries are to maximize mana regen and cooldown reduction while adding as much damage output as possible with a 9/0/21 tree. Mana regen on kassadin is a crucial part of playing him successfully. Without mana, he's useless so i feel that this mastery setup is non-negotiable.

I choose Ghost and Exhaust for kassadin because Flash is unnecessary with his ultimate ( Riftwalk) (in my opinion) so Ghost makes it easy to catch enemies or escape, especially if low on mana and Riftwalk is unavailable. Exhaust is my next one because of its decrease on the enemies speed and damage output, making it much easier for early game kills in lanes, and in late game team fights. And as said in my other builds, i feel most comfortable using these.

Your first skill out of the gate is going to be Null Sphere, for a great early game harassment. With the ability power runes that we use on this build and your first item, this should do awesome amounts of damage to enemy champions, especially mids (seeing as they are usually pretty squishy). Level 2 is going to be your Force Pulse to add that extra damage to your harassment or help in getting an early game kill with its slow. Nether Blade is next to regain that mana that you have been using. After lvl 3, i like to max out my Force Pulse as soon as possible to deal as much damage as possible. And of course get your ultimate Riftwalk whenever possible. When you get to lvl 6, it gets much easier to achieve kills. Your skill combo is going to be Riftwalk / Null Sphere / Force Pulse / Nether Blade. BE CAREFUL WITH YOUR ULTIMATE Riftwalk, the more you use it in a short period of time, the more mana it consumes, and you will run out pretty fast. After Null Sphere is maxed i like to max out Force Pulse next, and still get a point into the Riftwalk whenever possible. Nether Blade is last to be maxed.

Amplifying tome
To increase that early game damage and help get you some quick kills

health Potion
To aid you in staying in the lane for a longer period of time

Hextech Revolver
Starting towards your first full item, adding some ability power, and spellvamp to keep you in the fight.

Sorcerers Shoes
For that movement speed that everyone always needs, and magic penetration, since your a mage after all.

Will of the Ancients
Another increase in ability power and spell vamp, with an aura that your whole team can use. Underused item on kassadin in my opinion.

Catalyst the Protector Catalyst the Protector
To help address Kassadin's mana dependency and give you some extra hp to increase your survivability.

Blasting Wand
Increase your ability power and start towards your next full item.

Archangels Staff
Greatly increases ability power and mana to address 2 of kassadin's most important attributes. This item is a must on kassadin.

Sheen
A small increase in Mana (thanks shrubninja) and ability power with an awesome passive. Due to kassadins low cooldowns, this item helps to deal out alot more damage in your bursts.

Lich Bane
In my opinion, one of the best items on kassadin. An increase in magic resistance, large increase in ability power, and a passive that cant be matched. This item is groundbreaking when it comes to your damage output.

Banshee's Veil
To give you some much needed survivability and a passive that could potentially save your life.

Rabadon's Deathcap
A huge burst in ability power to top things off. If the situation calls for a different item, this item can be swapped for one of more importance.

This build should give kassadin a great amount of damage output, and a sufficient amount of mana to continue that damage output. Remember that kassadin is not an easy champion to play, and that he is one of the squishier champions in the game. Doing your best to stay out of the mess in team fights and as far away as possible from enemy champions is crucial to playing a successful kassadin. I hope you enjoyed this build, good luck, and have fun. Feedback is much appreciated, But i encourage you to try this build before commenting.

If you liked this guide, check out my others if your interested.