Kassadin is an overlooked character that people would call OP, or "Easy to Use"(Noobs-wise). He only seems OP because of the fact that he's overlooked. No one notices him, nor do they care, they'd rather focus more known champions and try to counter them instead. They don't see the need to counter Kassadins(in non-ranked), though don't worry, they will, eventually...and eventually means in maybe 4 minutes through Mid lane. Kassadin dominates Mid like he's a ranged-dps-tank-nuke-mage-assasin-bruiser-pusher champion(WARNING: Results may vary).

Well, more over on how this build is going to be played, this is my build more towards using Riftwalk Offensively. This requires a watchful eye of the Riftwalk CD and Skill Duration. Using this skill more effectively can bust out kills much better than the simple Null Sphere to Force Pulse Hit-&-Run combo. So let's get started.

The Skill Sequence is the same for all Kassadins; Null Sphere - Force Pulse - [RiftWalk]] - Null Sphere.

But this time, the Skill Sequence will be put in use differently. We're going to squeeze in much, much more damage from Riftwalk. When you're ganking from behind a wall or brush, if you see that you have time before ganking, Riftwalk around yourself till you have as many stacks on it as possible, but careful. Make sure you have enough Mana to use Riftwalk again, including a Null Sphere and a Force Pulse.

The Carrier: "Oh, it's Kassadin, don't worry, he already used his Null Sphere and Force Pulse, I don't think he's going to do any more- HOLY ****! CHEESE IT!"

Why would that happen you say? Well, let's do the math shall we?

Riftwalk's damage is 120 at lv3, now, let's say with you 3000 Mana, you stack it out 5 times. That's 120+120+120+120+120 used on the floor. Now that in mana is 100+200+300+400+500, a total of 1500 mana. Oh shet, you used half your MP bar right? Not a problem, you dont need no more then 790 Mana. Because that 790 Mana is now 1300 Damage EXCLUDING your Bonus AP/Pen. By this point, if you have 3000 mana, Expect to have around 500 AP, which is now +350 Null Sphere, +400 Force Pulse, and +200 on Riftwalk. That's now 1300+350+400+200=2250.
Q: Is your Enemy dead?
A: If you're talking about the Cockroach I stepped on, then...Yea, it is.
Just Recall to town, restore your MP, and back out for some more.

Pros :
-Deal exquisite team damage in little time.
-Continuously battle in fights even with Null Sphere & Force Pulse in CD.
-Overlooking the damage will haunt back at those foolish to do so.
-Extremely Dangerous and Spammable when defending Nexus/Nexus Turrets(Abuse Fountain Regen.)
-Stackable before entering ganks/team fights.

Cons :
-AoE range is small.
-Requires Close-up Combat for usage, leaving you vulnerable to Dpsers.
-Mana Usage will require you to Recall Many times(Counter this with "Crest of the Ancient Golem).
-Mostly useful after level 16(Level 3 Riftwalk).

Masteries are usually the same, You want the Spell Penetration, and the usual MP Regen/CDR for him. But you can go into 2 roads. Whether you want a longer(30%+) Blue Buff duration, or flat CDR with Exp Bonuses. It will depend if you like to get buffs, or you're the type who doesn't think about it(It's okay to have a little of both as well).

Now, about the runes. Most Kassadin's I've seen like to have Greater Seal of Replenishment and Greater Glyph of Mana. But this is not going to work out for a Kassadin who'm spams Riftwalk. We're going to get Greater Seal of Vitality and Greater Glyph of Knowledge. This is my personal choice because you're using Riftwalk, its range is pretty melee-ish compared to your Null Sphere and Force Pulse, even though Health is an issue for all Kassadins, including Mana. But here, we're taking Per level runes solely by the fact that you get more. Kassadin is a Mid-Late game champion, You wont be fighting before level 7-9 anyways. Might as well get the per level bonuses. Last is Greater Mark of Magic Penetration, Magic pen is a must for Mages, so let's leave it at that.

Well, We can all agree that Cleanse is the most recommended for Kassadin. Escape spells like Flash wont be that great for you already have one. If you're silenced, neither Riftwalk nor Flash can help you. Cleanse is a must whether for chasing or escaping. I've seen others who would recommend Heal, but I don't see the point in it. "It's great for baiting and helping you live with that extra health". <--any champion can do that, okay? So forget that and lets bring out that sexy Cleanse.

Now, why I choose ignite is because he does need that little extra fire power, Lich Bane is a late game item, so without that, your immense damage still wont be enough to kill people if they have too much health. Oh and yes, this build will be so that you wont have to chase far even if your target is a Full Health Champion. Ignite is always used when you tell yourself, "****, my food's getting away". bust out that Ignite, punch in a few melees if u need to and wahlah, you get an easy kill.

Teleport is a nice skill and all, fly around the map, defend lanes, unexpected ganks, ward/mushroom/jack-in-the-box jumping, but....the Ignite will get you a load of kills. more then Teleport anyways.

Well, The build is simple. Start out mid lane with a Meki Pendant, and 2 Health Potion. Patience in your lane with smart last hits and harrass can have you Recall at lv7 and buy a straight up Sorcerer's Shoes, and maybe a Tear of the Goddess if you had Firstblood or had some successful ganks. I also personally like to buy several Health Potion every time I Recall, it's a good save against poisons, ignites, and such else. As the one who will be getting most of the kills, it would be best if you're the one to buy wards as well, if they have a jungler, ward up your buffs and make sure your team is ready to gank when one is spotted. You can also protect your laners by warding up the river. As the one who KS's their kills, you should atleast spend Ward-money yourself.

Next we have Archangel's Staff. Mana Regen, MP, AP, what else does Kassadin need? a "Must" for a first item. Stack up that MP passive.

Here's where it gets good. Rabadon's Deathcap...oohoohoo~ a good 350 AP total by now, this is the point where your Null Sphere and Force Pulse damage doubles. A pity we cant have the Ring's active, but more AP is fair enough.

The item paths changes now. Lich Bane - Banshee's Veil - Rod of Ages
The item you need will always vary based on your opponents. Health is usually the issue, though Hard Nukers could also be the problem, so choose your item to your best opinion. Get Lich Bane only if you're completely destroying, and you want to destroy even harder. This item has now given you an extra skill-sort of. pop out a null(500 avg.), a force pulse(600 avg.), then swing that baby and finish em off(100dmg+AP[400] avg.). Though if they dont die, this is why you got your Riftwalk to finish them off(dont forget you can Ignite as well).

Now, some other items that are extremely useful can be :

Quicksilver Sash - Another cleanse, MR, good use.
Void Staff - Anti-MR Tanks, AP, needed late gaming/counter players.
Abyssal Mask - MR, AP, Reduces enemy MR, that's too good to be true.
Zhonya's Hourglass -...and so The Koolaid man said: "Oh YEA~"


Though ofcourse, if you want to have some fun....this should be your build.

Sorcerer's Shoes
Archangel's Staff
Rabadon's Deathcap
Archangel's Staff
Archangel's Staff
Archangel's Staff

So the guide is just about done, though let's discuss the gameplay of ranked games.
Here's what happens when you play out your cards right, and abuse the stacking damage of Riftwalk:

Low on Health? Full on Mana? Don't you worry, stroll your way into a bush, and rift their *** when they don't see it comming. Kuke them with it by rifting behind them, and they'll chase the other way, after that, rift them again.
Are they ranged? They'll nuke you if you're spotted? Not a problem......uh....yea that's a problem, and so wut? rift walk your badboy out of danger and home sweet home, after you've healed, say farewell to your extrodinarily safe Nexus and be off on your killing spree once more.

Just remember to Recall even if you dont have the early game money for sorc boots, you're better off slow on gold then dead. risking these things can get you killed unless you know 100% how your bait plan is going to work out, or whatever you're doing.
Stacking Mejai's Soulstealer is good and all but you'll have to study all of your opponent's gameplay, you might get a feeling that "they'll focus me", or "they might be good after they get some levels/gold", "they might tank my ***", whatsoever. careful that you dont waste your precious gold. The quicker your items are built, the better you can F*** up their Gold income.

But ranked isn't so easy, Kassadin gets easy kills, but don't ever get cocky. One slip-up, expose your gameplay, and they'll bait your *** all day.

That's all I can say about that for now.

Okay, Let's talk about......

HOW TO ANTI-KASSADIN!
Guys! It's Very Obvious!

Kassadin has such immense Damage output that after he attacks you, and you're falling less than half health, you consider retreating. If you got your correct items, if he's low on mana(say, 199 or less), you can take him down. Kassadin's weak point is actually his offence. After the Burst of damage, he has nothing to do afterwards for a good 9 seconds. He will consider after 4 seconds, when his Rift CD is done, whether to retreat, or continue to chase you. wether he does or not, it wont matter, you have 9 whole seconds to damage him.

You see, Kassadin is the type of champion who deals the damage any other champion would in a long, set period of time, but all together at the same time. for example, think about an anivia, but doing the Frostbite twice, yet takes a while because of the CD <-that's Kassadin's Damage, but he does it all together in exchange for a longer CD, meaning he goes completely vulnerable to any kind of attack after his burst. One verse One-wise, he can'y use his Force Pulse after one use(most likely) including that Null has a big CD. There's your chance, you a Dpser? Well that's perfect. Go Pwn his *** k? Have fun.