Revised and Updated - 01/21/11
*Rewrote and added info. for "Summoner Spells" Section
*Split "Items" Section into two, and revised my view on some items
*Added "Builds" Section
*Added "Runes" Section
*Added "Skill Breakdown" Section
*Rewrote "Gameplay" Section

I've been playing Kassadin for quite a while now as my main. This guide is just based on my experience playing him and toying around with different builds. So this would be my first build posted on Mobafire, any comments or suggestions would be appreciated.

Summary

Ideal Build:

Combo:

The runes I use for my build is very much like that of any other caster... but it works!

Greater Mark of Insight
Probably the best Marks for any caster. Extra magic penetration... what's there not to love?

Greater Seal of Clarity
Kassadin without mana is nothing more than a free kill. Mana regen is extremely important for you as it's your damage and survivability at stake.

Greater Glyph of Focus
Cooldown is imporatant for any caster and mroe for Kassadin. Short CDR Riftwalks are the key to escaping certain death.

Greater Quintessence of Fortitude
A bit of extra HP never hurts and is beneficial for your early game. No one likes being First Blooded in the matter of seconds.

Cleanse
I cannot stress this enough, Cleanse is probably the best Summoner Spell for a mobile champion like Kassadin. You need to jump in and out of a fight unscathed and the only way that's possible to is to avoid CC. If you're CC'ed, just Cleanse and Riftwalk out of there, simple.

Other Spells for Your Choosing

While I have taken Teleport for my other Summoner Spell, it is mainly just due to preference and other spells may be just as good. Here are some with a bit of advice.

Teleport
This is always a great spell to get around the map quickly and efficiently. It has many uses including: needing to recall to shop/heal, defending turrets/allies, farming large minion waves, etc. Not necessarily a must but recommended.

Flash
Not as useless as many may think. This is something not many people may expect and will make you a lot harder to kill. If you get into a sticky situation, just Riftwalk and Flash away and no one will be able to catch you. It also allows you to play a lot more aggressively early game.

Clarity
This spell is generally not needed if you have Greater Seal of Scaling Mana Regeneration and the correct masteries. However, if you haven't yet purchased these runes or too low of a level, Clarity would be very helpful.

Ignite
While not the most preferable spell, it could be a good First Blood tool. Though, for the most part (Mid-Late Game), you should already has enough firepower to take out your desired targets. It would be preferable to free up this slot for another spell.

Exhaust
A "catching-up" tool as well as a 1 vs. 1 tool. It can turn the tides in a 1 vs. 1 fight or make a difficult chase possible. Not greatly useful but does carry its advantages.

Heal
I have looked around and there are methods to baiting enemies with heal. In my opinion, it's really a waste of a slot and Heal is quite pitiful later on.

Ghost
You have a much better built in escape mechanism and "double-flashing" is also more effective. Not recommended.

Unmentioned Spells

Any spells that aren't mentioned aren't of great use to Kassadin, but if you feel that they fit your playing style, feel free to take them with you.

All these items as I see are absolutely mandatory for Kassadin and shall be described below.

Mejai's Soulstealer
You might be wondering... why this before boots? Well I say it's because you will likely be calling at around level 6 for your first shopping trip. Level 6 is the beginning of your highpoint and you can easily manage to obtain a few decent stacks before laning phase is over. Keeping the stacks isn't terribly difficult either thanks to Riftwalk.

Sorcerer's Shoes
This gives you a nice early magic penetration bonus. Great for harassing your laning opponents who have some pesky magic resistance.

Archangel's Staff
The Tear of the Goddess component of this item is terribly important for a mana abuser like Kassadin. You very much need the mana regen to keep mobile and dishing out damage. Getting caught without mana is basically your funeral.

Lich Bane
You want to maximize your damage output for every burst. This item will add another damaging component to your already devastating combo so your can erase more health from your enemies. (Fun Fact: Lich Bane proc. deals physical damage so remember to use Nether Blade for better damage).

These items can be used to fill the void in your inventory. Here are some descriptions of each listed below (items in red should be taken special note of):

Banshee's Veil
As with every other guide, I'd have to agree. Kassadin's is very vulnerable to CC and Cleanse will not be able to deal with every one of them. If you see that the opposing team is packing heavy CC, be sure to add this crucial item into your build.

Rabadon's Deathcap
A pure AP booster item. Why not want more AP?

Void Staff
A very useful item against teams with high magic resistance. Just when they thought they saw the last of you...

Doran's Ring
For those that do not have the suggested Runes and Masteries, Doran's Ring makes a great starting item. The extra bit of health, mana regen, and AP is very much needed.

Ionian Boots of Lucidity
An alternative to Sorcerer's Shoes if you do not yet have Greater Glyph of Cooldown Reduction.

Rod of Ages
While not a recommended item, this is for those newer players or those who dislike snowballing. Replace your Mejai's Soulstealer with this if necessary. [Note: this will greatly affect your rise to power yet allowing a bit more survivability].

Abyssal Scepter
Provides you with some magic resistance, AP, and an aura debuff. A great item overall but should only be taken against low magic resistance teams (for high MR teams, see Void Staff.

Zhonya's Hourglass
I still generally dislike this item. It roots you in place while giving you invincibility. For a mobile champion, staying in one place is never a good thing. It may save you once, but not twice.

Deathfire Grasp
A mage's only item to counter tanks. If the other team seems to tanky, grab this for good measures.

Stinger
A very situational item where both teams are pushing for victory. Cheap and gives you some CDR. Should be sold as soon as you see fit.

Miscellaneous Items

Elixir of Fortitude
You might wonder why a magic damaging champion might need this. Simple, it gives you an instant HP boost. It is almost like a Heal to go but for 250g, plus it provides benefits for as long as it's active.

Elixir of Brilliance
Some extra AP and 10% CDR. What you really wnat from this is the 10% CDR since you won't be reaching 40% CDR anytime soon.

Elixir of Agility
In most cases, useless on Kassadin. But, if your in a game where it's push to win, grab this for some extra quick pushing.

Sight WardSight Ward
Every good player's best friend. Ward their jungle, dragon and Baron. If you have Teleport, you can easily rack up kills by messing with their jungler. They'll never see what hit them.

Core Build

Final Builds

These are just examples and your build will usually differ between games.

Ideal


Beginner (Lacking Appropriate Runes and Masteries)


Strong CC Teams


Tanky Teams

This is where I'll be explaining the uses of Kassadin's skills. This isn't a description of what each skill does as you I'd already assume you know how each skill functions.

Null Sphere
This is what you will want to max out first. While it does less damage than Force Pulse at level 5, it is your key harassing ability [and it silences too!].
Harassing/Last Hitting
Use this to last hit minions that are out of your reach or to harass your laning opponent(s) down. Try not to spam this skill at early levels (ideally after level 3) as your mana will suffer. They key is to get them low enough to be killable at level 6 yet not low enough for them to tower hug or recall.
Interrupting/Silencing
This is where Null Sphere truly shines. It can be used to interrupt any channeling skills ( Karthus) or to silence any of those casters before they start nuking your team ( Annie).

Nether Blade
This skill's only benefit is to provide Kassadin with an endless supply of mana as long as there is something to attack (syncs quite nicely with Void Stone). Use it whenever possible in order to charge your Force Pulse. It also works well with Lich Bane as the proc. is dealt as physical damage (no one will know!). Other than that, it isn't worth investing in until level 13.

Force Pulse
This is your highest damaging skill in AOE form and it slows! (*gasp*) The only drawback to this skill is that it requires 6 charges (6 nearby skill casts) after every use to use again. In teamfights, this isn't a problem but laning against opponents who rarely use their skills can be problematic. Save your Force Pulse during laning for dire situations (escaping) or when going in for the kill. Later on, it can be used to farm large minion mobs. (Tip: Use your Froce pulse stacks to be aware of nearby enemies. If you gain a stack without seeing anyone use a skill, know an enemy is in your vicinity).

Riftwalk
This is the reason why Kassadin is such a great champion. A built in Flash on low cooldown makes you mobile like no other.
Laning Phase
Once you hit level 6, start saving your Force Pulse stacks (at least 4 stacks) while harassing them to a comfortably low health (see Null Sphere Harassing/Last Hitting). When you feel comfortable, Riftwalk into them, use Null Sphere and then Force Pulse (autoattack them a bit if they didn't die yet). This should be a guaranteed kill and 2 stacks for your Mejai's Soulstealer. Rinse and repeat.
Escaping
At the sign of danger (where you are sure you are going to die), Riftwalk out of there immediately even if you have to leave your allies to die. Do not stay there unless you are sure you can save them and get out alive, your stacks are important. Remember, continuous Riftwalks will lead to mana starvation in no time. Always be sure you have enough mana for at least 2 (100 + 200 = 300 mana).

Burst Combo

Before anything, you need to know Kassadin's combo inside and out.
1. Riftwalk
2. Force Pulse
3. Lich Bane (if applicable)
4. Null Sphere
Try to Force Pulse immediately after Riftwalk to slow them and be able to get your Lich Bane attack off. Null Sphere has a farther range so it can be used last. Do not try using Null Sphere first as it may scare them away losing a potential kill.

Laning Phase

This phase will generally determine the outcome of how well you perform for the rest of the game [without massive amounts of kill stealing... don't do it]. In laning, try to gain a few stacks (anywhere from 5+ is ideal) from killing your opponent(s) or getting assists (see Riftwalk Laning Phase). If you finish your laning with 0 stacks, things are going to be problematic for you. Tower diving shouldn't be a problem for you after level 6. Combo them and run away and you should suffer 1 or 2 tower hits.

Teamfights

If you're entering mid-game with no stacks, try to fulfill the role of an anti-caster and just be a nuisance to the enemy casters. Please do not kill steal your allies who are **** well, they will be better off than you.
If you're entering mid-game with around 5 stacks or more, great! You have one role in teamfights, killing the enemy squishies. I've seen many Kassadins run around during a teamfight doing nothing while letting the carries slowly tear a team apart. Use your combo (see Burst Combo) on a squishy and the Riftwalk out of there immediately. If you get CC'ed, remember to Cleanse and get out of there before they focus you down.

Assassinating

You're an assassin! Remember that. Use the terrain and Riftwalk to your advantage. Hide behind a wall and jump on unsuspecting enemies. If you get the drop on them, they will usually be scared ****less and start running. Screw with their minds so that they become paranoid and tower hug, not bothering to push.

Chasing

Kassadin is an amazing chaser and barely anyone will be able to get away from you. Using you combo and Riftwalking, Null Sphere once more will usually finish off your targets. If not continue chasing them, but be careful, don't be lured into a trap. If you suspect a trap or doubt you can finish your target before you over-extend too much, back off. One kill isn't worth one death.