I chose this build because it makes sense. Kassadin is a nuker and assassin. Getting that early Blasting wand boosts your AP a lot. Really there is no need to get a doran's ring. He has a good 500 health in the beginning. So I took Sapphire crystal for the extra mana to harass. And don't forget too get hp pots they can longer your sustainability in lane. I chose to take the catalyst for the mana and health so u can lane longer. I also find that the lichbane is a great item for Kassadin because of his ulti using so much mana. The void staff can be replaced with the Abyssal Scepter if the other team has a lot of AP. I almost always get Abyssal Scepter when facing Annie of Veigar. I don't really see a need in getting rylai's because his E slows them anyway.

I took the original Flash and Ignite as my summoner spells of an obvious reason. Almost every Ap champion takes them. Flash helps in the beginning because of his ulti using so much mana.

I chose this skill order because it lets your E ability get maxed out as quick as you can. At lvl 9 i start to roam a bit. I roam with kassadin because he has a slow and a teleport. Alowing him to initiate esaily with out any time for them to react. Usually ganking top a few times will make them loose xp and be behind in there lvls. The reason I dont get riftwalk 2 at lvl 11 is because Kassadin doesnt have the mana to use riftwalk frequently. So getting null sphere gives your more damage and longer silence duration.

I took the my runes so I am more successful early game. I took the flat AP quintessences so i dont need to buy a doran's ring. Notting getting the doran's ring allows me to get Sapphire crystal for more mana as I explained before. I got Magic pen marks for more damage against tanky champions and junglers. I took mana regeneration seals so i can lane longer.