Kassadin is my main champion and he is very good at carrying at ranked games after he is level 6. His early game is really bad but he excels at late game.

There is a place between dimensions and between worlds. To some it is known as the Outside, to others it is the Unknown. To most, however, it is called the Void. Despite its name, the Void is not an empty place, but rather the home of unspeakable things, horrors not meant for minds of men. Though such knowledge is lost in modern times, there are those who have unwittingly discovered what lies beyond, and they have been unable to turn away. Kassadin is such a creature. He was once a man forced to look upon the face of the Void and forever changed by what he saw. Once a seeker of forbidden knowledge, he discovered that what he sought was something else entirely. He is one of the few that has found his way to forgotten Icathia and lived to tell the tale, following the scant breadcrumbs hidden in ancient texts.

Within a decaying cyclopean city, Kassadin found secrets of the kind that he will never share - secrets that made him quake with fear at the visions of things to come that were thrust upon him. The power of the place threatened to consume him forever, but Kassadin took the only route available to him in order to survive - he let the Void inside him. Miraculously, he was able to overcome the alien urges that went with it, and he emerged as something more than human. Though a part of him died that day, he knows that he must protect Valoran from the things scratching at the door, waiting to get in and visit their torments upon the world. They are only one step away... something to which the appearance of the abomination known as Cho'Gath attests.

If you look upon the Void, you can't put it behind you. If you look upon Kassadin, he is probably already there

Kassadin should only mid against ap carries. if he doesnt face an ap carry he will not win mid and will not carry. just silence and kite with kassadin and you should win mid against most ap carries.

Pros

good antimage
good late game
good against squishies
can kill entire enemy team
good kiter

Cons
bad farmer
blue reliant
bad early game

Flash
dont use this because kassadin already has a flash
Cleanse
I use this because kassadin already has an escape move and if he isnt cc'd then he will escape most likely
Ignite
I use this to add extra burst
Revive
no...
Rally
no...
Clarity
can use this if u dont have mana regen runes
Teleport
can use this instead of ignite if you dont find ignite usefull
Exhaust
can use this instead of ignite
Heal
don't...
Smite
extra farm if u really want it but...
Fortify
cant really tower dive kassadin after level 6