Hello,
Its my first build. I dont want write a lot yellow, red, blue text, because my english is not very good. I will write few things how I play.

I use this skill build, becouse w isnt very usefull when you got clarity. I use only it on late/mid game.
About build:
First, i'm take:
Clarity:because kassadin use a lot of mana, even in mid game i useing in a lot.
Second, I take:
Teleport:Becouse it help on early game to fast back to lane, when you got low hp or mana and clarity got long cd. Btw its very usefull for gank's

Glyph: I take magic penitration runes for mark, because its very usefull in full game not only on early.
Seal: I take mana regeneration runes, because like i said before, kassadin use a lot mana.
Glyph: I take cd reduction, because more cd reduction, more and faster damage you can do.
Quintessence: I take cd reduction per level, because like i said before, more cd reduction, more damage you can do.

I choice 9/0/21 build, because I think the best for kassadin. On a lot another guides i saw, that people choice 9/21/0. I think they take more armor build, becouse they think kassadin is very squishy, he is, but he got very usefull Ultimate skill, with he can asily run away. So my offer 9/0/21, becouse you gain mana+cd reduction.

First i take: 1.Catalyst the protectorI think its the best item for early game, you get hp+mana, and after lvl you gain restore hp and mana who will help a lot, with this item and luck, i made a lot double kills on 4-8lvl of this item passive.
2. Boots, i take sorcerers shoes, becouse it gives magic penitration wich helps at start
3. When i dont buy a litle time nothing i take Needlessly large rod, it gives a lot abilty power.
4. Now i look what i need, more ap or more deffensive item, i take Banshee's Veil or Rabadon's Deathcap.
Its my core build, when most of time i take items like i showed before.

Early game:
On early game you must play with kassadin very carefully, becouse he dont have Ultimate to fast run away, and my choice of summoner spells not Flash or Ghost. I dont care if I take mid, or 2v2 lane, i always try hit without taken damage Q spell, and hitting last hits to minions, when i get Catalyst the protector and have 5/6 lvl i try to get some kills.
Mid game:
Mid game, my tactic try to gank with teleport or running enemies summoners, or kills people on team fight, if i got ap item i try to kill 1v1. On mid game you can be not so carefully, becouse you got Ultimate.
Late game:
Late game, thats starts a lot fun with kassadin, if you have good items 200ap, you now can made a lot of damage, most summoners will run from you. In late game i made a lot quad kills, double kills. And I just love Kassadin ultimate, its help a lot run away from ganks(main rull, jump through the walls), or catch running low hp enemies summoner.

First of all on summary I want to say sorry for my bad english(if you see very big english mistakes, write me i will fix), second it's my first build, so tell me what's is wrong, I will try to fix it.

Kassadin is very fun champion with you can have a lot fun, like jumping around nexus when you try to kill, and you are catch by enemies champion, or made a lot damage. I tryed to use another builds from this website, but all didint work for, only one and old, its Kassadin: Badass Rift Ninja, but with my this build i play better.
Proof:I will add more later.