Personally, when I first bought him, I was upset with my choice of IP. I had a choice to either buy Shaco, Xin Zhao and Kassadin. A week or two later, Kassadin is my main and if I ever want to rip **** up, this is the guy.

Before you go ahead and tell me that this guide is rubbish and will get you nowhere in ranked games, at the time of me making this guide I'm level 23 and am still getting used to LoL itself but I thought this guide was worth making because using this EXACT build I have had more flawless games (with 10+ kills) than I have with 1+ deaths.

One more thing before you read this guide, Kassadin is a NUKE character. This means he can absolutely unleash on a character but after that you need to get the hell out and wait for your cooldowns to wear off.

Prepare to rape :)

The Goods:
- AMAZING at harassing (This is mainly early - mid game)
- Probably the best nuke character I've ever used/seen
- Can virtually walk through walls
- Has potential to stay laning till level 10+ without going home
- With a full build ability power is 900+

The Bads:
- Squishy
- Mana dependant early to mid game (but can be countered)
- In team fights, PEOPLE FOCUS ON YOU

Personally I would consider Kassadin easy to play but hard to master. You need to learn that in order to survive and farm well you have to learn how to harass (annoy the enemy champion or champions you are laning with). Fortunately for you, your Q spell [Null Sphere] will silence your enemy making it so they can't cast ANY abilities, also inflicting a neat little amount of damage ;)

Firstly for beginners I'll explain his moves:
Q [Null Sphere]:
A ball flies out doing damage and silencing the target hit (can be cast on minions and champions).

W [Nether Blade]:
At ALL times (even when activated) your regular attacks or "Auto Attacks" will restore mana. When activated it will make your basic attacks also do a set amount of magic damage. So when you hit a target you will be in theory hitting them twice, with both magic and attack damage.

E [Force Pulse]:
A cone of damage is sent out dealing the same amount of damage that a Q attack would do except this can hit multiple targets and slows them.
Unfortunately this spell can only be cast once 6 champion abilites have been used near you. UNtil then you can't use the spell. The ability can only be stored once meaning that after you use it you have to have another 6 abilites cast near you (These include yours, team mates and enemies).

R [Rift Walk]:
This is what makes Kassadin who he is. A low cooldown flash that deals damage. Flash over walls, even the wall between the jungle and your base. Learn to master this because this is your way to escape from harsh times or to gank or even to kill a champion.

This should come naturally as you read through Kassadin's abilities but just in case :)

Say you're on your turret, hugging of course, and an enemy champion is getting greedy, you're either going to want to finish them off or scare them off (making them teleport home enabling you to farm).

Heres how it would go, you would start from a bit back, and then start to glide in and lead with Rift Walk right next to or on top of the champ. After you're going to want to hit them with both Q and E, starting with E [Force Pulse] if they're already retreating. With Null Sphere already being a lot higher than your other spells, and just being sturdy overall this WILL damage them. At level 6 (when this combo is first avaliable) it easily deals well over 200 damage making them retreat to their turret or to safety almost instantly.

Sometimes you will be close to them and CONSIDER using your teleport, if they are within auto attack range, but be careful with riftwalk!! Despite a cooldown of less than 4 seconds, its those 4 seconds that make the difference between them escaping and you getting a kill or even you getting away if **** gets sticky, again its just your better judgement. This time, lead with your E. This can go either way for you, it can secure an escape and just be a quick harass or it can be the beginning of their end. If they want more, hit them with your Q silencing them, stopping them from doing any abilites to you, VERY GOOD against people who use abilities to deal most of their damage. After this hits them, you can activate your W and start laying into them. As you near the end of the combo, based on your own personal judgement you want to hit them with your teleport to finish them off (if you think its worth it) OR teleport away.

As with most characters, words can't exactly explain by themselves, you have to experiment, and even failing combos will help you learn.

TL;DR,
1. Lead with R --> Slow with E --> Hit with Q --> pursuit with W.
2. Being chased, E Backwards --> Lead with Q, instantly activate W, turn around and unleash --> Unsafe? Teleport out // Safe? Teleport infront of their path and attack.

Sorcerer's Boots:
They help you run fast and do magic penetration (makes your magic hit higher through your opponents magic penetration.

Mejai's Soulstealer:
Champion Kill: 2 Stacks
Champion Assist: 1 Stack
Maximum of 20 stacks: 1 Stack = 8 Ability Power
The default is 20 Ability Power maxing out @ 180 Ability Power.
Downside is when you die, you loser 1/3 of your stacks (Die with 20 stacks, you will have 14)
As a bonus, when you have 20 stacks you will also have 15% cooldown on your abilites :) Much consider this item a gamble, but I find it extremely difficult to die as Kassadin unless you get stunned and ganked, even then I play extremely safe when I'm using Kassadin.

Rabadon's Deathcap:
This gives you 140 Ability Power and has a passive which gives you a 30% bonus to your ability power. Unfortunately having 2 of these doesn't mean you'll have a 60% bonus.

Archangel's Staff:
Gives you ability power, mana regen, mana and a neat little passive which will end up giving you a nice extra 1000 Mana + ability power based on how much mana you have!

Lich Bane:
After casting an ability, your next auto attack will hit 100% of your ability power. Late game 800+ AP. Get the idea? Also gives you movement speed and ability power :D

Elixir's:
Very late game items when you can't get anymore, it just sends your ability power to 900+ :)

A tip is to shag your turrets. As dirty as this seems, Kassadin doesn't opperate much other ways using this build. Naturally, champions will be EXTREMELY scared of your turret. If they do try to turret dive you, this is when you rift walk out.

Ignite will be used to kill people when they're getting away and on low hp when you have cooldowns. Flash can be used for both escaping purposes and catching purposes (This plus Riftwalk can easy make you catch an enemy, or easy save your life, use it wisely).

As I reach the end of my first guide, I hope this build comes in handy, please don't go too hard on the ratings, and remember this is for newcomers who don't even know what ranked is.
Have fun temeporting through Summoner's Rift ;)

P.S. I didn't include Runes because I don't even have enough myself to buy any so I can't tell you what is best but I would have to take a wild stab and go with Mana Regen/Capacity and Ability Power.

EDIT [04/09/11]:
Changed the guide because of how noob the build was :/ Added Cleanse because its useful both late game and early game instead of just early. Added Elixirs & Archangels.