Intro
This guide is built off of Kassadin's need for mana regeneration and his squishiness. I find that after completing Shurelya's Reverie I rarely run out of mana to cast spells, but before that i would run out at in the worst of situations. All the items other than Archangel's staff and Rabadon's Deathcap address his squishy state. OK i made him not as squishy; this does not mean that you can get up in their face whenever you want, he has his moments. You need to lane with someone until you get your ult at the very least.

Pros/Cons
Pros

Amazing Damage output mid-late game
Is your team QQ'ing about an enemy mage? you can handle it
Tower diving like no one else could after getting your ult
Looking Badass

Cons

Sooo squishy early game
Skill, Kassadin takes a large amount of skill to learn to use effectively
Very Vulnerable to certain CC such as Suppressed, Stunned, Taunted, Fear, and to a lesser extent Snared.
Becoming the focus of team fights late game (i'll cover how to counter this)

Runes
Honestly, this is a recommendation, go with whatever runes you want.

Masteries
See Runes section

Summoner Spells
Heal and Ignite are nice but not the only summoner spells viable on Kassadin. I've used Cleanse with amazing results. Clarity is viable early game but loses usefulness late game.
Which ones should you not use?
Revive-... i can' stress how unimportant this spell is, you only take spell if you plan on dieing, that shouldn't happen too often with this build
Smite-You aren't jungling, enough said.
Rally-WHO THE F**K USES RALLY?? i mean c'mon riot.
Flash-This is already built into your Champion.


Items
Items and my reason for buying them:
Doran's Ring- Exceptional starting item, Health, AP, and Mana regen
Sorcerer's Shoes- Magic Pen. Increasing that important damage output (I have tried Ionian Boots of Lucidity and concluded that they are viable, but i prefer Sorc. Shoes more)
AA Staff- Max Mana on each spell cast, Mana Regen, AP in large quantities (the Mana on spell cast is amazing, Kass is extremely ability heavy and therefore will treasure that)
Soul Shroud- Health, Mana Regen, Cooldown Reduction (reducing squishiness, Mana Regen is a huge focus in this build, and CdR will always help Kass, that ult becomes spammable like no other)
Shurelya's Reverie- It's Basically another Soul Shroud, but because CdR is a UNIQUE passive it won't stack if 2 Soul Shrouds are bought.
Rabadon's Deathcap- Bulk AP... enough said
Rylai's Crystal Scepter- Health, AP, and that Passive makes it so much better.

Playstyle
Mindset- This is one of the most important parts, you must be Aggressive and Loose, meaning, you will trade blows with them. You must go on instinct. You shouldn't let a few deaths get to you, especially in your first few games. After saying rely on instincts i feel that it is contradictory to say that you must think about things too. If you're chasing someone always keep in mind the chance that they are baiting you into their team mates.

Early- This is one of the only times when you actually get up in their face, try to attack them a few times, this will make them fear you, harass them with null sphere and force pulse when it is ready to use. This is going to instill fear. they will be scared to fight you. If they are not scared, show them that they should be. I'll provide a good list of champs to lane with at the bottom of this guide.

Mid- You can really get aggressive now, you can chase, and tower dive like no one else. In team fights make sure to silence their heaviest mage, get as many enemies in your force pulse area as possible. If they start to retreat, don't let them. Chase them using rift walk to catch up and force pulse to slow. If for some odd reason your team starts to retreat, use riftwalk to make a quick get away.

Late- if you're doing your job right, they will be focusing you in team fights. Avoid this by rift walking away, then move back to fire null spheres and force pulse. This usually makes them forget about you. If you are out matched and trying to make a get away remember to rift walk over walls to make it easier. At the end of late game Kassadin tends to fizzle out, the enemies are catching up on their items, meaning you have less of an advantage. You can still get kills; it's just a little harder to do so.

Tips

Nether Blade can be used as "free" force pulse stacks becuase it has a very low mana cost
Rift walk>Null Sphere>Force Pulse is a deadly combo, especially against casters
Null Sphere can be used when farming to gain force pulse stacks

Laning Partners

Blitzcrank- Lot's of CC to help get early kills
Pantheon- Not a tank by any means, but it works quite well
Rammus- Tanky, and can sponge damage for you
Mordekaiser- he can be good, but can sometimes steal your kills

Please comment and rate so can get an idea of what's wrong Thanks and have fun using Kassadin