Hi! My names Orcyish, and this is my first guide on MOBAfire.
I have been playing Kassadin alot recently, and realized that he had alot of potential as a AS/AP hybrid with his Nether Blade damage, in addition to his passive Void Stone which will make you even more dangerous in team fights. So here it is, my idea of playing a new kind of Kassadin ^.^ this is just an idea I had, hope it works out for you. ENJOY!

Above is my Ideal runes setup. However, there are many, many options in this department. here i'll go over a few.

Greater Mark of Magic Penetration By far my favorite mark, without these your damage will be reduced noticeably endgame. However not the only viable option.

Greater Mark of Attack Speed These marks will help you allot early game, before you get all of your items, Or it will help you if your building the tanky route, because you pick up a rod of ages and Rylais rather then attack speed items.

Greater Seal of Armor These seals i recommend, because you will be up close in combat, being melee and all :) and these will give you some nice survivability, and i always take these if im in a solo lane. ESPECIALLY middle, so that they cant do as much damage harassing you with auto attacks.

Greater Seal of Attack Speed These will help you early game, however, while armor is in play, i would go with Flat armor seals.

Greater Seal of Scaling Mana Regeneration These are viable, and these are what i would take on a basic AP kassadin, However, if your having serious mana issues you can take these (if you cant get yourself a blue buff)

Greater Glyph of Magic Resist I always take these if i go middle, the extra MR will allow you to survive a burst from a Veigar or Leblanc (or at least not take as much from it :D)

Greater Glyph of Scaling Magic Resist These will be the best endgame, the MR Will make you able to live through an AP characters burst much better. However, i do prefer flat MR for middle.

Greater Glyph of Scaling Ability Power These will give you some really nice damage endgame. Take these if you want some extra endgame damage.

Greater Glyph of Ability Power These will give you some very nice early game damage. There very nice to have, but i would take force over these, just because the AP difference they offer endgame.

Greater Glyph of Attack Speed These are really 'meh' glyphs. I would take MR or AP over these anyday.

Greater Quintessence of Magic Penetration My personal favorite quint. The magic pen is extremely nice to have and will help you alot throughout the game.

Greater Quintessence of Ability Power A very strong quint to have. These would have to be my second favorite. They give you a significant damage boost throughout the game, but especially early.

Greater Quintessence of Attack Speed Very viable, but i still would prefer magic pen. to attack speed runes, in any slot.

Greater Quintessence of Health My personal favorite tanky rune. With 3 of these, you get a 68 health bonus right off the bat. And if you also get the Veteran's Scars mastery, that's 98 starting health, which is more then the basic doarns item starts you out with.

Greater Quintessence of Magic Resist Very nice, i would go with these if you have to go mid, and you know your fighting someone like Annie, Veigar, or Leblanc. (That is if your going draft pick)

Greater Quintessence of Armor Not a huge fan on kassadin, i would take Health over armor in a quint slot.

Boots: If you want to build Glass Cannon get Sorcerer's Shoes, the magic pen. this grant you will increase your damage alot. Although if you have Attack speed marks/ Quints you may want some Berserker's Greaves. This attack speed boost will help alot early game, but if any of them build MR, Make sure to switch out later. If you want to survive over damage, get either Mercury's Treads or Ninja Tabi. I perfer mercury's treds ESPECIALLY if they have a high burst caster, but if they don't, and its about 3/5 AD feel free to get tabi.

Malady: Really nice item, fairly inexpensive, nice attack speed and AP boost, and its passive is like the casters Black Cleaver. Get this 1st, sell it later if at lest 2 of them have 130+ MR and get Void Staff.

Wit's End: Huge damage boost, and nice survivability increase. Stacking MR plus the 45 bonus magic damage, this is a must get. If the game drags on and your fully built, you can replace this with a hextech gunblade, which will give you nice Spellvamp and lifesteal, which= survivability.

Nashor's Tooth: nice Ap boost, mana regen attack speed and huge CDR, nice item. Get this if your building glass cannon, and if not, choose this OR madreds bloodrazor, depending who they have on their team.

Madred's Bloodrazor: OP :D~ Attack speed, AD and 4% of their health! thats a HUGE boost. Get this if your building glass cannon, or like it says above get this or nashors tooth depending on their team comp.

Frozen Mallet: Very nice item to get, 700 health is a huge bonus, a nice ammount of AD, and of course its passive, your already highly mobile, and this passive wont let any enemy escape you.

Rod Of Ages: Very good health, mana, and AP boost, get this early if you decide to at all.

Guardian Angel: My favorite defensive item on Kass, especially with this build. Gives you one last chance to riftwalk to saftey. You can replace this with any other defensive item however: I would choose Banshees vial if they have a karthus, or ezreal so you can block their ult and get away. But you can also get thornmail if their AD heavy, a frozen heart again, if their AD heavy.

( Email me with ideas or suggestions, or comment, i appreciate it :) )

This is how i skill up, leveling Null Sphere at 1 so you can trade harass, Nether Blade at 2 because you might not have Greater Seal of Scaling Mana Regeneration, and Force Pulse at 3 so you have an actual slow. I then continue to get Riftwalk at 6,11,and 16 ( of course) and leveling my w second, because its your main source of damage on auto attacks. I then choose to level Force Pulse third because its slow % increases, although you can level Null Sphere or Force Pulse third. ( or a mix]]

So your skill priorities should be Riftwalk--> Nether Blade--> Force Pulse--> Null Sphere

Flash is a must have. Your not fast enough to need ghost (however ghost is very viable) and it will give you one last chance to escape if riftwalk is on CD.

Ignite is a very nice damage boost, and you will absolutely need it against a WW, mundo or another one of those annoying healing champions.

Exhaust is nice, but not needed when you have your slow and Frozen mallet. But it does lower their defences, your call.

Teleport is a nice spell, but leave it to a tank to get this. Although if you need to, go ahead :)

Heal isnt good on Kass, because you already can escape almost anything

OTHERS: CANT THINK OF ANY THAT ARE GOOD xD
(Tell me if im missing any that works on him. its late :P)

Thanks for reading, hope you like this build and give me +1 :D
Im open to suggestions and tips, so email me at Orcyish@gmail.com or comment. I do have alot of work to do, so consider this the "Public Beta" (I have no icons or anything yet)

Also wanna give a shoutout to jhoijhoi, i read the Making a guide, guide :) it really helped me understand code a bit, and was just really informative.
THANK!
~Orcyish.

~ 5/30/2012: Added my first code, and changed the runes section a bit.