Hello my fellow LoL players. Im here to introduce you to my way of playing Kassadin, specifically on how to counter other ap carries in mid. With this rune/mastery setup and through situational items, you can laugh at the LeBlanc who is used to oneshot people but now can't do anything to stop you. This is my first guide, so please be easy on me :)

Pros
+ High damage output mid game / late game
+Personal flash on ridiculously low CD
+ Enemy AP carries cant do **** to you
+ He's BS
Cons
- Weak early game until you get lvl 6
- Countered by AD mid
- If enemies are playing right, they will focus the **** out of you

In my opinion, this is the main mastery tree for most if not all Casters. This provides some nice damage increase for Kassadin, which is useful because my runes dont provide any AP. With Kassadin, i usually take 9 points here to support my defensive set of runes, but with all the other AP casters i play, i usually take 9 points in utility instead of this. If you really wanna be an immovable rock in early game, you could even try 9/21/0 masteries. This way you eliminate the most of your early game squishiness. I love this mastery tree on basically every caster and jungler, but with Kassadin i like to take the mres and health from the defense mastery tree to support my tank'ish runes.

Kassadin's passive. This is one of the main reasons Kassadin is considered an anti-caster.
You take 15% reduced magic damage. This with the MRes glyphs and the 21/9/0 mastery tree, it makes you very tanky against magic damage dealers.
Kassadin's Q. This is your main harrasment/nuking ability. It packs some really nice damage and offers a huge silence, which really is one of the reasons why Kassadin owns AP carries. You want to max this first.
Kassadin's W. Fairly useless on AP Kassadin, but I suggest you to take on rank at lvl 8 to charge your Force Pulse and maybe even earlier if you want to gain some precious mana from smacking things with your badass sword. You want to take one point at lvl 2, 4 or 8, and max it last.
Kassadin's E. This is used as a part of your R > Q > E combo, which is the main combo for nuking people. This packs nice damage just like Null Sphere and slows the enemies, potentially preventing some frustrating low-hp escapes. You want to max this after Null Sphere.
Kassadin's R, aka ultimate. This is the skill that makes Kassadin as badass as he is. It's potentially a flash on ridiculously low cooldown, which also deals some hefty damage. If you use this repeatedly, it's mana cost will increase rapidly (just like Kog's Living Artillery), but it will also gain more damage. If you have the Blue Golem buff, which you should have always when possible, It's possible to stack Riftwalks so it deals so much damage that you will be able to oneshot squishies with your combo. You want to take this at levels 6, 11 and 16 like with all ultimates.

Now for the items.
I almost always start with Boots and 3 Health Potion, so you can easily dodge enemy skillshots, and since they nerfed the range of Kassadin's Null Sphere I really think that the movespeed is necessary. If I happen to have some low range, low mobility laner against me ( Ryze, Warwick), I might just start off with a Sapphire Crystal so i get a really healthy manapool for spamming Null Sphere. I then immediately rush catalyst the protector to [[rod of ages, so i get a huge manapool, some precious AP and a good amount of extra HP. After this you should start building towards a Rabadon's Deathcap, or if you are totally destroying the enemy team, Mejai's Soulstealer. When building Rabadon's Deathcap, you should prioritize the Needlessly Large Rod but if you aren't doing so great, you should buy 2 Blasting Wands to get you going, because you need the second blasting wand for Void Staff anyways. After Void Staff, you have two choices. If you want to keep yourself just snowballi'n on the enemies' ***es, buy damage items like Zhonya's Hourglass and Abyssal Mask. If you suspect that the game is going to be somewhat even, always fit one defensive item (ex. Banshee's Veil or Frozen Heart in the build.) Your endgame build could look something like this:


Versus AP/CC Heavy teams
Or

Versus AD Heavy teams.

If you love spamming Riftwalkand always end up with no mana when the need arises, you could fit this into the build. If you really want this item i suggest you to take the Tear of the Goddess right after you finish Rod of Ages, and then finish your Tear to an Archangel's Staffafter you complete your Rabadon's Deathcap

Kassadins main function in teamfights is to nuke squishies down if they overextend and clean up the survivors after the battle. NEVER initiate with Kassadin. You'll die in seconds and get maybe half of their carry's HP down with you. Not worth it. Wait for your tank to initiate, and when you see that one of their squishies is out of position, Riftwalk> Null Sphere> Force Pulse If you are fed/well farmed, this will devastate any squishy carry, because the amount of damage you do is just ridiculous. Protip: If you see that their carry has an Banshee's Veil, Riftwalk in their face! Riftwalk is mainly used for utility, so you can easily pop the spell shield from Banshee's with this without losing much of your damage.
Also, always remember to use your Null Sphere on people channeling their ults ( Fiddlesticks, Katarina). This can turn the tides of the battle in your favor.