Yo! My Name is BlaqueJaque and have been playing Kassadin as my main since about season 3 or the start of season 4 - don't really remember. Anyways, as time has passed I've been adapting the remakes on Kass and exploring different builds to create the perfect Kassadin throughout the whole game. Right now I'm at a peak, since I've never seen him this OP as right now. The good thing is, people don't really play him no more because he is hard to play, but once you know your ins and outs, you will be god

Back to the Kassadin build
As of right now, I like to play aggresive with him, but I've always felt like he could need more sustain rather than mana or AP, because his Nether Blade gives him 4-8% mana when hitting minions or other monsters and a massive 20-40% mana when hitting champions. So, when you are trying to escape enemy and you are out of mana to do more Riftwalks, just try and hit enemy with W and Riftwalk the hell out of there. Meanwhile when engaging in teamfights, you don't need that extra mana from Archangel's Staff because you got Nether Blade - instead you want items like Rylai's Crystal Scepter. Yes, I know Rod of Ages makes you sustainable, but that's still no excuse for buying items like Archangel's Staff. You can make a lot more multi kills if you got sustain. Anyways, I'll go more in-depth on how to play Kassadin later.

Pros

High mobility
Massive single target burst
Easy last hit on minions once you got your w
Shield against AP champs
Silence against channeling (kata, malz, Warwick)
Gank pro after lvl 6 - just riftwalk in - burst - riftwalk out (simplified)


Cons

Squishy early game champ
melee champso you might struggle against mages and marksmen
Hard to play you can either use Riftwalkthe right way, or you can use it the complete useless noob way

Mark
As an AP Carry, the only Mark you need is Mark of Magic Penetration.. Because you are an AP Carry.

Seal
As for the seal though, you want either want Seal of Health or Seal of armor, because as a melee AP mid, you want the sustain. I usually go with Seal of health because I quite often go against an AP champ - just to make sure the seals ain't wasted early game. If you are up against AD mids like Zed, Yasuo and Irelia, pick armor. It helps a lot early game.

Glyph
On the Glyphs I would put Glyph of Magic Resist, to sustain AP champs , like LeBlanc, Veigar and Brand - because your Q shield won't be enough against their massive burst. Only if they're full AD you should consider changing them to Glyph of Scaling Ability Power.

Quintessence
This one is easy. Greater Quintessence of Ability Power. Pls don't ask why.

Under progress

The Dark Seal: I recently started to begin with this item and it shows out to be really efficient against champions I know how to play against, because it stacks on kills and assists (read the item description). It's an OP item because you can have about 100-200 AP after just 10 mins. So, yeah.. If you're unsure about who will win the early game mid lane, start with Dorans Ring, otherwise I recommend The Dark Seal.

Warding Totem: I bet you know what this item does. It's more how you use I'm concerned about. As Kassadin is good at single-target bursting, he might struggle 2v1. That's why you want to ward ahead of where you want to go, meaning fx. If you want to play top mid side, ward the top river brush, and try to put the 2nd ward in their jungle.

Mejai's Soulstealer: It's a given to rush this item, if you're going with The Dark Seal. It gives you extra base AP and mana AND it still stacks ofcourse.

Corruptin Potion: Instead of going with the regular health pots, buy the Refillable Potion and build into this item after Mejai's Soulstealer. It gives that extra ignite feel, when finishing an opponent.

Rod of Ages: This is a core item on Kassadin. I know you might think you want to rush this before all else, and I think so too, but wait those extra minutes, before getting this. It's worth the wait and if you know how to play Kassadin right, you will agree - I hope. Rod of Ages give you extra health, AP, and mana AND it stacks (read item description). After RoA, you are basically no more squishy in the early-mid game. On the other side you are OP when single-target bursting.

Sorcerer's Shoes: It's a given to buy magic penetration boots when you are playing Kassadin, because he's an AP champ and the opponent, especially the tanks, will buy Magic Resist.

Rabadon's Deathcap: This item is a core item if you've hauled in kills. Not only giving you 120 AP, it also increases your AP by 35%. Let's calculate this: Mejai's = 145 AP + RoA = 245 AP + Rabadon's = 365 + the 35% boost = 493 AP!!!. And this is without taking the runes into consideration, so you will have well over 500 AP after just three items.

Rylai's Crystac Scepter: This item gives you 100 more AP and 400 more HP, which means you have more burst damage and more sustainability. Other than that it makes your Q and W slow your target by 40% for about a second and your E by 20% for also about a second. This means, that you don't have to waste you R on chasing only one opponent, as you can save it for the next one.

Luden's Echo: This item works like the Statikk Shiv passive. Instead of giving you attack speed though, it gives 100 AP, 10% movement speed, and applies it's passive onto up to 4 opponents AND the passive applies Rylai's slow passive onto targets. Nice.

Void Staff: I once saw this item as a core item but nowadays I only purcase it, if the enemy is buying a lot of MR, as it make you ignore 35% of the opponent's MR. And it also gives you 80 AP

Iceborn Gauntlet: If you are up against AD champions like Yasuo, this item is crucial. It gives you 65 armor and 500 mana, which means you can rush this item instead of Rod of Ages (I don't do that often though). Figure out how good your opponent is before rushing this. If you do rush it, it will give you god defense and when you burst the opponent you do extra physical damage on your W or after using your Q.

Abyssal Scepter: Buy this if you are having a hard time on lane against an AP champion. It gives you 80 AP, 60 Magic resist, and it reduces enemy's magic resist.

Frozen Heart: Buy this if you are getting ganked alot from AD champs. It will reduce their attack speed and you receive 90 armor and 400 mana.

Nashor's Tooth: If you are having one of the easiest games in your life and you want to end it quickly, buy this to take down minions and towers fast as it gives you 80 AP and 50% attack speed.

Lich Bane: Since Luden's Echo was introduced with it's movement speed, I've never felt the urge to buy this item. It gives you 80 AP, 250 mana, and 7% movement speed. Not my type of item, but sometimes I need the extra mana to escape champions like Nasus, Volibear, and Shyvana.

Elixir of Sorcery: Once you've finished your built, buy this for extra AP boost.

Oracle's Altercation: You are an assassin. Use this to be sure u can hide in THAT specific brush.

Vision Ward: I ALWAYS buy this. helps you fend off enemy jungle and to see hidden champions like Shaco. And to see if enemy has warded brush. So it's both an offensive and defensive item.