Not Updated For Current Season
okay now lets tell you the sense of this build
Saphirecrystal: first you buy the saphire crystal its really usefull, because you can spam your first ability and suck your enemys with this item and clarity, so you got no mana problems at start. Second reason is you can combine it direkt to,
Catalyst the protector: with this item you wil suck your enemys if they got a spam ability too because the passiv of this item is to get live and mana if you get level up so 1more item to get out of the mana problems.After this you have to buy,
Mejai's soulstealer: because you need ability power and can do kills at start without AP because kassadins abilitys are really strong with AP too. After this you have to complet the,
Rod of ages: because you have to get full power of rod of ages with getting this item in inventory 15 minutes. And in top of that this item will help you with all your lifeproblems and if you didnt got stacks already it helps you with a bit Ap.
Tear of the goddess:you need this item to spam your ultimate and have to get the mana allready for archangel's staff later.
Boots of mobility: with kassadin it doesn't matter if you buy as late your boots because you can teleport with your ulti so you can catch people easy and run away easy. Why boots of mobility? because you have to gang the people on other lanes to get manny stacks. And with this boots and your ultimate you can get there really fast.
Archangel's staff: now its time for the archangel's staff because you got enough mana on your Tear of the goddess. With this item you can convert the mana of archangel's staff and rod of ages (2,5% of its amount) into AP. So you got now 205 ap + mejai's soulstealer
and you got enough mana for everything. Now to the especially of this build, i tought hmm kassadin can teleport really fast to enemy and its an Ap champion so why i dont should take;
Lich bane: with Lich bane you can teleport to enemy damage him 40% ap + basedamage of ultimate, hit him for 100% ap+ base AD-damage (because of lichbane) (you can activate your netherblade for much more damage, onlyest usefull use in this combination) , null sphere 80% AP + basedamage of null sphere (lichbane is on cooldown) (you have to cast null sphere before your force pulse because enemy could use a skill to teleport or anything like that), force pulse for slow, deal 80% AP+base damage of force pulse and than with speed of lichbane you can run to him (you are faster than he because he is slowed and you got 8% more MS than he) and hit him for 100% AP+ base AD damage. After this you start this combo again (most times he is dead after the first time). with one time this cobo you deal, 400 % AP and 200% of it penetrats his armor/ magic resistence (1400-1800damage) , + all the base damages (800-1000damage). The magic resist is usefull too and the mana is really nice for archangel's staff.
Zhonya's ring: now you can buy the of all mages loved item, zhonya's ring you need this for 25% more Ap and the nice 120 pure ability power.
Archangel's staff: you can sell your boots now because boots aren't as important for kassadin because of ultimate. You should buy now a second archangel's staff. I know the effect of the 1000 mana don't work twice, but the effect of 2,5% of mana is converted into AP so you get 125 or more extra ap and you get 400 bonus mana for the first Archangel's staff too so you get 5% of 4000(mana) as AP (200ap + 2times the 50 ap of archangel's staff so 300 ap).
Archangel's staff: I hope, your mejai's soulstealer got enough stacks but if it doesn't got enough, you should buy 1 more archmage's staff and sell mejai's soulstealer, so you get 400 mana and you get 7,5% of your mana to your AP. So you got as much AP...
In the end you got: |(Archangel's staffs 7,5% of 4500=350AP + base-AP of Archangel's staff 3x50= 150 150+350= 500 AP | Lichbane 80 AP | rod of ages 80 AP|
Zhonya's ring 120 AP + 25% of all (25% of 780~200) 200+120=320 AP . So 320+80+80+500= 980 AP
you need the 4% aditional mana on the right tree for the archangel's staffs ( i didn't included it for summing up the AP )
and you need the middle tree for your defense because in my build is near to no defense.
you need much ap at start for getting stacks so i took the instant AP-runes.