The guide it a bit outdated. I'm planning on updating it within a moth.
I've come up with this build through playing, playing, playing and trying different things out. I'm pretty confident in this and can with over 500 matches worth of experience as Kassadin vouch for it's awesomeness.

This build is almost unstoppable in solo lanes. You'll be harder to face when you have Tear of the Goddess. Only a select few will be able to stand against you.

The thing that makes this build different from other builds is the fact that I use Strength of Spirit . It will make a lot of things much easier. Read the masteries section below for more info.
I hope people are willing to try new things. Because this is a great way to build Kassadin.

I will not go through what the abilities do. It can easily be learned by reading the ability tooltip and simply using the spell.

Mouse-over any items or abilities to see their tooltip.

All capital letter words are abbrevations. Hold your mouse over the word to see it's meaning.

"Why not choose utility?" You may wonder. The answer is pretty simple; you can't get both Strength of Spirit and Archaic Knowledge with 21utility. Those are the two most important bonuses availible to Kassadin in masteries. As long as those two are chosen the rest aren't all that vital. Getting Tenacity with the remaining points is the best choice if you have the mana-reg runes. If you don't, you can just spend them on Meditation and Utility Mastery if you want more mana-reg.

Strength of Spirit really is awesome. It gives 0,3% of your mana per second. So 1,5% per 5. 1000mana = +15hp/5. Every 1000mana you get a free Regrowth Pendant, worth 430gold. That's pretty ****ing good. Compare that to the 3AD from Brute Force which only is worth 124,5gold. at all times. I know it may be unpopular to compare the gold worth, but there's simply too huge a difference to ignore it.
Lategame you'll run around with over 50hp-reg/5. Even at lvl 1 you'll get +6hp-reg/5 with a Sapphire Crystal. Starting a game with 14hp-reg as Kassadin, without any hp-reg items is pretty nice. This mastery doubles your hp-reg early on and more than quadruples your hp-reg with the right items in lategame. Very nice to have.

Ardor is also helpful. When fully geared (with the example build) you'll get 19AP from it.

The only things you might miss in utility is the CDR and the mana-reg. The rest are expendable. I get both of those things in my runes.

1: Sapphire Crystal 2: 2x Health Potion 3: Tear of the Goddess

4: Ionian Boots of Lucidity 5: Doran's Ring 6: Archangel's Staff

7: Banshee's Veil 8: Void Staff 9+:Recommended items

You can throw in a Mejai's Soulstealer if you're confidient that you'll get kills, but I do not recommend it unless you know you'll do good.

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Laning Phase:

Try to stay mostly defensive and focus on lasthitting below lvl5. Don't be afraid of using spells to lasthit minions, but make sure you don't use too much mana. When you reach lvl5 you should begin to weaken your lane opposition. Harass the target as much as you can, but try not to take any unnecessary dmg. Keep harassing until the opposition has fallen below 500hp. When you reach lvl6 you should try to kill them (if below 500 ofc). Try to catch them unaware by Riftwalking in, then use your standard nuke combo. Before doing this you need to keep good track of your mana. If mid you need atleast 270mana. If laning with several opponents you should try to get the target when his partner is some distance away from him or have enough mana for another Riftwalk. If you can't get them/him/her/it below 500 before lvl6; below 550 should work at lvl7, but that'll require atleast 280mana. Don't forget to account for the extra mana costs when levling up skills! Always measure your dmg to make sure you can get the kill before going in! Make an assumption while taking their magic resist into consideration. If the enemy never gives you an opportunity for a kill just focus on keeping him away from exp and gold. That's much better than going for a kill when you're unsure whether you'll get it or not. Push the minions against the enemy tower if they recall or die, then recall yourself. Just make sure you aren't taking any chances. After that you can go gank or get golem if you think you can do it without losing a tower. If you're dominating the lane try to keep your enemy away from exp and gold by standing between him and the minions. This build is exceptional for just that.

A few tips for laning against someone with a spell shield:
(I.E: Morgana and her Black Shield, Sivir and her Spell Shield)
Try to lure them into using it by using the attack command on them.
Spam your Null Sphere and Force Pulse on them whenever they've just used the shield.
Get close to them, they have less time to react then. But consider the risk too ofc..

As always; I'd like to express how important it is to only lasthit when you aren't pushing (or pushing back a big enemy wave). You're simply taking unnecessary risks by pushing into enemy territory when you aren't gunning for their tower. Especially important against junglers. You should also only lasthit when you're trying to keep the enemy away from exp and lasthits (it's called zoning).

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Laning against:

Vladimir is really hard to face since he deals dmg and gets hp on each Transfusion, which also is instant so you can't scilence it in time. His combined HP gain and dmg is larger than most spells in the game while also having a very short cool down. He will come out on top of most spell exchanges. Pots will aid you greatly in defeating him. Try to damage him when Transfusion is on cooldown. It's much harder when he has maxed out Transfusion though.

Nidalee can be hard to face. She will probably dominate the lane before you get your Tear of the Goddess. After that you'll hopefully be able to keep up with her heals. It can be very even.

Sivir can be a big threat because of her ability to almost completely nullify your harass. She also has her Boomerang Blade which can make for some annoying harass. It's very easy to block your Null Sphere with Spell Shield because of its' slow missile speed. You'll have trouble harassing even a bad Sivir. Ask for ganks or switch lane if she gets too troublesome.

Morgana's pool can make it hard to protect your tower. She quickly destroys entire minion waves with it. Always make sure you have minions in front of you or that you can dodge her Dark Binding. Attack her when she gets close to the minions to lasthit, she won't be able to bind you then as long as your minions still are alive.

Tristana has really long attack range and can in the hands of a skilled player be very hard to play against. Don't let her Rocket Jump on you.

Laning against Anivia can be a pain in the ***. She's got Flash Frost which has both longer range than Null Sphere and stuns. Be very careful when going for minion kills, it's much easier to predict your movement when your destination is known.
Don't try to kill her if you're low and she's got her Rebirth. You'll need a few auto attacks after that to get the kill, but since you get aggro from the minions it's too risky to try that when you're below 400hp. It's sooo damn annoying to die from the creeps while almost breaking the egg. (has happened to me twice >.<)
Once you have Riftwalk you can jump over the ball and go straight to attacking her whenever she uses her Flash Frost. Make sure you're at a safe distance to avoid backlash when the scilence is over though.

I'm thinking that I should add a few more champs here so if you have trouble with anyone; tell me. I've pretty much faced everyone but a few new champs. The only champs Kassadin have trouble with are the ones that have longer range than you or can negate your harass.

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Teamfights:

Stay on the outskirts. Don't join the fray. Jump around killing ppl while not giving your enemies a chance to kill you. You operate outside the battle, assassinate key targets and never expose yourself to more danger than you can handle. When playing right you're actually more of a "Flippin' Ninja" than the ninjas are :). Do not use melee attacks unless you've got a death wish. You can ofc use them in the aftermath though.

Hold off using Riftwalk until you really need to go over something or need to get to or away from a point quickly. Using it too much will quickly starve your mana pool and make you useless.
Walk to up your target(s), scilence (disablers are prioritzed if they might focus you ofc), slow and then Riftwalk away if the area poses a threat. Only use Riftwalk offensively (teleporting into battle, instead of out) when you're sure that it's safe. Even when you know it's safe you should make sure that you're just inside Force Pulse range, not closer. Escaping can get hard if you go closer.

Try to scilence key disablers, supporters or damage dealers at decisive moments. Make sure they haven't already used their spells. Though it's hard being picky when you can't take much damage. Just don't attack enemies with high amounts of MR.
Rammus, Annie(only if you know you can do it safely. Annie with Pyromania Tibbers is lethal to Kassadin), Kassadin :), Malzahar, Shen, Soraka, Zilean, Fiddlesticks and Kayle, just to mention some.
Scilencing anyone when they're about to save or disable someone can make a big difference in the outcome of a fight. That applies to any fight.

Use passageways to your advantage. If you know they will pass through a small passage to go where they're heading; stand in one corner and try to hit as many as possible with Force Pulse. Just make sure they can't get you afterwards though.
You'll usually get opportunities to attack the enemies through walls a few times a game. Always make use of that opportunity. This is especially important with Kassadin because he never gets positioning problems.

Try to refrain from initiating teamfights before you've got Zhonya's Hourglass. You'll most likely end up in pieces. Though when you've got it; you can use it as a bait, to make the enemy team lose some power. If both teams seem to be waiting for someone to initiate you could do it and bait the enemy team into losing a disable or two.
Let me use an example of something I did one match:
Both teams were stuck at our tower, but noone dared to initiate. I don't blame em', their Annie had a Pyromania Tibbers ready. Strangely enough, some people don't like getting a big ****ing flaming teddybear summoned on their face. So I took a chance; I riftwalked in and used Force Pulse, then immediately used Zhonya's Ring(this was before it got removed). Annie used her Summon: Tibbers on me, making them lose a lot of their nuking power and then we just went berserk on their ***es. We killed all of them and got their Nexus right after that. That was an awesome match, very even and fun. I love being the one that carrys my game to victory, especially when my teammates have no BALLS (pardon my french).

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Throughout the entire game:

There's one thing I really can't stress enough. That Riftwalk is NOT a primary source of damage. While there are moments it can be used for extra dmg those moments are rare and only in1v1 situations. Riftwalk is for transportation and last resort damage. Like when Null Sphere and Force Pulse are on cooldown and an enemy is about to escape. Never use it on an enemy to initiate, unless ofc they are alone and have no way of killing you.
The best example I have is NinjAChurch. He uses it for damage all the time and does really well. The only reason he doesn't die all the time is that his enemies are totally incompetent. He would have died a ****load of times if only his enemies knew how to play.. (in that vid atleast) The "Make your own luck" part made me want to cry.. Those were among the most stupid manouvers I've ever seen a Kassadin commit. (not all of them though)
As for why; the problem with using Riftwalk for dmg is that you open yourself up for dmg. Should a squishy Kassadin give the enemy an opportunity to kill him? I don't think so... A good Kassadin attacks the enemy but never lets them attack him. (well, that's the goal atleast)
It doesn't get any more ninja than that ^_^

The golem buff is a great asset. Try obtaining it whenever you can safely leave the lane so that no tower will be lost. Having a Sight Ward in the bush behind the wall behind golem or in the river path bush (when on the top team) greatly reduces the chance of a steal or gank.

Play like you don't have Cleanse even if you have it. Never get cocky knowing that you have one free save from CC. That only leads to bad things.

Playing against a stunner or someone with a knocked airborne or stun spell is annoying. Be very careful and always be aware of their location. Knocked airborne can't be cleansed.
Warwick's Infinite Duress, Malzahar's Nether Grasp and Urgot's Hyper-Kinetic Position Reverse all suppress their targets. Suppression totally disables you, you can't even cleanse it. Always scilence them when you think they might use it.

Your standard combo can vary. It's usually best to use Force Pulse then Null Sphere. Because Null Spherehas longer range. If you're attacking an enemy with a "teleport" spell - like Tristana with her Rocket Jump - you should scilence first. For obvious reasons - you shut their "teleport" down when you scilence them. It also depends on if the enemy has spells that can hurt you or not. If there are hazardous spells involved you should always scilence first.

After getting Tear of the Goddess you should Riftwalk in the spawn whenever you get the chance to (like once before moving out, don't stay in the spawn to stack mana..). For some extra mana and charging your Force Pulse. When you finally get Nether Blade you should use it too. Even when not in the spawn. 25mana is so cheap that it's worth it as long as you aren't low on mana.

Obvious to some of you, but slowing or scilencing enemies chasing a teammate might save his ***. Always scilence the disabler before slowing, the other way around might get nasty. Never sacrifice yourself for others if you have a killing spree going. That's like donating gold to the enemy team..

Scilencing an enemy that is channeling a spell is very good for everyone. (except for them ofc xd)
Nunu & Willump's Absolute Zero is rather nasty if left unscilenced. Karthus's and Fiddlesticks's ultis, if you can make it in time. Fiddlesticks's Bountiful Harvest. Pantheon's Heartseeker Strike. Malzahar's Nether Grasp. To mention some.

Kassadin is great at baiting. With his insane mobility you can lure an entire team around in the forest indefinetly as long as you have mana left. Make them believe that they can get you, but always make sure that you have the situation under control. Never let a disabler catch you unaware. Coordinate your baiting efforts with your team and you could achieve awesome results. It's hard to bait smart enemies though >.>

Use Zhonya's Hourglass whenever you end up in a bad situation. I'm gonna name some moments it will help.

• When you've got a dot on you that will kill you. Make sure that you're a safe distance away from enemy champions though.
• When you're about to be hit by a disable in a tricky situation. ( Rocket Grab and Cryptic Gaze among others)
• When you have Riftwalk on cool down and is about to be hit by a ton of dmg.

Also lovely for baiting as mentioned a while back.

Nether Blade has 3 primary uses.

• Charging your Force Pulse and Archangel's Staff.
• Increasing dmg with a Lich Bane proc (and making a proc ofc).
• To deal more dmg to minions and monsters.

Always remember the Force Pulse charging. It can make a big difference sometimes. I've gotten a few kills by using Nether Blade to gain the last Force Pulse stack in a critical moment.

Try to keep close to walls whenever you can. It makes escaping much easier. Especially important when exploring brush. Remember to move away a bit when facing champs like Poppy though. Her Heroic Charge can be painful if it hits.

Ambushing enemies through walls can be great, but it's important to be aware of how many enemies there are at the other side. Riftwalk into several enemies is bad, so very bad for your health.

Killing through walls is pretty nice when you get the hang of it. If you have vision of an enemy sticking close to a wall where he doesn't have vision of you, there is a golden opportunity to use your scilence+slow combo.
This is very useful when there are several enemy players(or a tower) in the vicinity and a Riftwalk would be too risky.
Two good spots to use this tactic are; the walls near the inhibitor turrets and the wraith spawns. Even though this is a rare opportunirty, I have gotten a few kills this way.
While there are champions that have abilities that are more suited for these kinds of things( Ezreal for example), Kassadin is still decent and definitely able to do it.
You can take this one step further. If you know you can go in over a wall and kill someone but there's lots of enemies on the other side. You can go in, kill and use Zhonya's Hourglass when you're about to be hit by something. By the time the stasis is over your Riftwalk should be off cooldown so you can just Riftwalk out again. Be wary of other teleporters though.

(Too long; didn't read)

Stay defensive until lvl5, Strength of Spirit should keep you healthy enough to get some minion kills.
Harass with Null Sphere as often as you can at lvl5 and up.

That results in one of 3 options:

• No mana = recall.
• They recall = quickly spam spells on minions to push yours against their tower then recall.
• They get low (below 500Hp) = try to kill. Don't get greedy though, it's better to stay and farm exp than to die from a tower.

All paths lead to either going to gank, keep farming mid or to get the golem buff.
If you're dominating the lane try to keep your enemy away from exp and gold by standing between him and the minions.

As always; I'd like to express how important it is to only lasthit when you aren't pushing (or pushing back a big enemy wave). You're simply taking unnecessary risks by pushing into enemy territory when you aren't gunning for their tower. Especially important against junglers.

The golem buff is a great asset. Try obtaining it whenever you can safely leave the lane so that you won't lose a tower for it. Having a Sight Ward in the bush behind the wall behind golem greatly reduces the chance of a steal or gank.

Focus on dealing damage and then Riftwalking away. Riftwalk is not a primary source of damage.

Try hitting many targets with Force Pulse in teamfights, but remember to avoid disables. Attack after they've been used or when the only disabler can be scilenced.

Nether Blade has 3 primary uses.
Charging your Force Pulse+ Archangel's Staff, increasing dmg with a Lich Bane proc and to deal more dmg to creeps. Always remember this. It can make a big difference sometimes. I've even gotten kills by using Nether Blade to gain the last Force Pulse stack in a critical moment.

Try to keep close to walls whenever you can. It makes escaping much easier.

Use Zhonya's Hourglass when you're about to be hit by a ton of damage or when you're about to die from something. Just like Cleanse it can be used to avoid disbles while removing the dmg too.

Try to scilence key casters on the enemy team. Don't sacrifice yourself doing it though, you're also a key caster :3

Baiting is a great tactic when you know how to. It is harder to make them take the bait when you're Kassadin though. Most ppl just give up the chase.

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Lastly, if you have any questions, corrections(feel free to correct my spelling too) or suggestions; comment. I'll try to answer them whenever I get the time to.
Credit for the pic goes to SilverChaos for removing the logo on that chinese piece of art.