This build is, for me, the best way to play kassadin. I play this and consistantly do very well. He has nice CC, nice damage, and an ult that makes him *almost* unkillable, once you master it. The build takes the core elements from the #1 rated kassadin build on mobafire (Badass Rift Ninja, by Killraven) and mixes it in with some of the best bits from the guides that arent *quite* as sucessful, but are still good. The main difference in this build is that you do not build a lichbane. You will NOT be doing much autoattacking. You WILL be riftwalking around team fights throwing in slows and silences where needed, picking off squishies and saving your own *** again.

For runes, Magic pen and CDR is essential. This should be your Red and Blue respectively.
For yellow, I normally take mana regen per level just to make you capable of throwing in some more harrassment earlygame, before you get your mana pool. HOWEVER, AP yellows DO work. If you feel like having a bit more damage, switch it up.
Quintessences are a choice between flat AP, or flat health. I recommend starting out with fortitude until you become competent enough with kassadin to go for the extra damage with the AP.

The right masteries are actually not as essential for kassadin, you MUST take 9 points in offence for the magic pen, but the rest is optional. 9/21/0 can work just as well and in some cases better then 9/0/21, equally, within the utility tree, you may opt for the extra mana instead of greed, or if you feel you arent likely to be grabbing blue a lot. Its all prefference. The main reason I take utility over defence is the movespeed and CDRs.

Summoner Spells - Teleport is essential if you are going mid(Which you should be). You dont need ghost of flash, you have the worlds greatest escape/chase move already. Combine that with cleanse and you are prettymuch unkillable. Unless a nocturne or tf ults you, but hey, you have silence and slows, and you still have that escape move. I would reccomend always escaping through jungles as you have walls there to riftwalk through and more chance of losing somebody who is faster then you. In conclusion, the masteries may be prefference, but the spells arent. Follow this guide for success.

Using teleport: Mainly use teleport as a way to shop, and get back to your lane instantly from early to mid game. You can later use it to try gank, which is especially useful if your team puts a ward in a bush near their tower, and initates a fight. Wait until they commit, then riftwalk in and finish them off. As a side not, without lichbane, kassadin shouldnt really be trying to backdoor. He doesnt damage the turrets THAT much, and although he is very capable of escaping andybody who comes to try their hand, its often better to save teleport for more cruical times.

Using cleanse: Pretty self-explanitory. If you are in the thick of a fight and suddenly you become stunned(for example), dont worry. Cleanse and riftwalk out. Even if you are still fine for hp, riftwalk out. After using a CC move, players tend to go mad and they will usually chase you a little bit while your team beats on them. if nobody is moving away, walk back in and throw some spells at people :)

I start with a Mana crystal and some HP pots, and build it into a tear and boots as soon as is possible. The extra mana is lovely. Once you have the tear, dont bother upgrading it to the staff for a while, you have more important things to buy.

Boot choice: Most guides advice sorcers shoes. I dont, I say get Ionian boots of lucidity, for the CDR, and make up for the lack of MPen with a void staff later.

Mejais: Although this is now largely frowned upon by most players as it is conditional on you getting a lot of kills/assists and not dying, as well as drawing a lot of enemy attention to you, in most cases, I would still advice you to get it for KASSADIN only. I rarely get snowball items on other champs, but kassadin is so hard to kill, and he has a couple of nice AoE moves, your stacks will build up in no time and stay built up. HOWEVER, if you feel your team is not doing well, or you are being heavily bulied and think that in that particular game you wont make much use out of a soul stealer, give it a miss. Get another item with some defence instead, i.e. zhonyas hourglass, though dont get it until after your rabbadons and void staff.

Take the middle lane. Play passively, try to last hit when you can, but make THEM think that they are dominating you. Make them play less cautiously. Ofcourse, throw in some silences when you can for a little harrasment, but dont go in for a kill until you have your ult. Once you do, hang back until your slow is ready, then wait until they waste one of their spells on a minion or whenever you think is a good time, riftwalk in, silence and slow AND RUN BACK. you may have done a lot of damage, but dont get greedy, your time will come. They are likely to stay in the lane since you ran away and were so passive earlie on, and a well timed, second combo is likely to finish them off. Dont be too afraid to towerdive for a kill once you have your ult, but do it smart. know they arent going to stun you and kill you, or heal and kill you, or fortify and kill you. I usually let my minion wave go in first, then WALK up to them, and throw my spells at them, and riftwalk TOWARDS them if they are not dead yet, or riftwalk out through the jungle if they did die (the jungle avoids more turret fire)
Bottom line, dont riftwalk in to dive. you wont be ablle to get out for a short while and the turret aggro will likely hurt you bad.

Once the laning phase is over and the teamfights start, your role is to stay out of the thick of it, but still help all you can. Silence those who need to be silenced (save it for anybody with dagerous ults) use your slow as a nuke to hit the most people you can, and riftwalk around the fight to catch that miss fortune off guard, the one who was, until now, safely just throwing damage at your team like candy. Once the teamfight is over, hopefully your side has won, there may well be a few people running away. Chase them, slow them, silence them, kill them (this is where you shine). EQUALLY, if your side is losing the teamfight, you can use your spells to support a teammate who is low on health. put yourself between the chaser and the chasee, and throw a slow at the chaser. Silence him. Often, you can turn one of your teammates from being killed into a nice 2v1 gank where you come out victorious. Dont count on this though, you are mainly just trying to ensure they get out alive.

If you have chosen mejais, and you are stacking it up nicely, the other team will try to focus you. use this to your advantage. Bait them if you can. A kassadin using his ult to get away doesnt usually be concieved as a bait. you can use it smart also, riftwalk thhrough a wall so that the team has to go round a pathway or through a bush to get to you. It just so happens that your team are waiting in that bush, and they didnt expect it because they thought you were somewhere else. If you arent baiting, hang back. never initiate, as you will lose a lot of health and be too scared to damage their team effectively. (Another probelem with mejais, is that people will abandon their team to protect stacks, when their presence could have won them the team fight). You DO want to protect your stacks, but your team should be doing a pretty good job of protecting you. don't be too scared, as you are a vital cog in your team.

Another note, it is usefull to pick up blue whenever you can for the CDR and mana regen. Make sure nobody else needs it more though.

*(A note on farming) once you've got your needlessly large rod, you should be able to take down entire waves with your slow. do it. And when travelling to and from places, throw a slow at the wraithes too. if there is a huge minion wave incoming to one of your undefended side lanes? get there and end it. you are a fantastic farmer at this point. you will start off slowly, so farm as much as you can later to make up for it.