Not Updated For Current Season
I have always played AP Kassadin and it worked just fine, but since last patches I have started to think about creating a hybrid build, as Riot created Manamune, remaked Melady, and Kassadin has quite high base attack dmg.
Ok, lets start with masteries. I put 23 points in offence, because i'm focusing on attack dmg, and using skills as a support, not main dmg's source. The most important is to get 15% magic penetration, 10% crit dmg and ofc last offence skill.
From Utility tree, i put 3 points in mana regen and 4 points in max mana.
I take red runes for crit chance. why crit chance insted of armor penetration? Because in this build this is ONLY source of crit chance, and I dont want to waste 10% crit dmg from masteries, however I haven't calculate what (crit chance or penetration) generate more dps.
Blue, Yellow and Quintessences for Attack Speed (about 20% of additional attack speed)
Exhaust- this kassadin fights similary to Master Yi, but do not have such speed boost, so, sometimes our force pulce slow and rift walk is to little to kill the enemy. Its also good spell agains other physical dps champs.
Teleport- well, its just cool ;p
I start with mana crystal and trying to get tear as soon as posible, as Kassadin needs mana a loot. We have no mana= we cant use Rift walk = we are dead.
After getting tear buy Berserer boots and Melady, this will give u nice att speed boost, and additional AP and dmg from new Melady.
Mid / Late Game:
When we have enough attack speed, we can complete Manamune, which gives pritty high attack dmg and rises our mana pool.
Next 3 items depends on the situation, if u die frequently, buy Crystal Scepter 1st, if not, u can consider Lich Bane, or at least shen before scepter.
If u played AP Kassadin before, get ready for diffrent play style. This is not 'rift + quick combo +run away' build. Use W spell just after rift and slow, and keep hitting as long as its posible. Most of dps casters will now have any chance against You.
Always remeber about Exhaust against melee dps!
This hybrid build is rather late game, but after 10 kills u should feel its potential ;)
Give it a try! ;)
(Sorry for any language mistakes. Feel free to write comments, opinions and corrects ;)