Hello and welcome to my quick kassadin guide witch will help you to win the game on mid without any problems.

pos and cons
+ funny to play
+ easy to master
+ counters almost every one
+ after 6 you can not be ganked
+ 2 ccs
+ easy awesome ganks after 6 on top and bot
+ by my oppinion one of the best assassin
+ can kill even with low on health

- quiet mana hungry
- squishy
- item dependent
- easily targetable without ultimate
- hard early game because he is melee
- king of bans in ranked games

i personaly take ignite foreasy finishing and barrier to survive any dirty situation or if you want take flash i dont recommend you any other, because they just dont fit on kassadin

Void stone (passive)the magic dmg reduction is one of the reasons why hy is so good against most of ap carries, but i dont see and use of increased attack speed.

Null sphre has the same pic like baron buff but its also really nice and easy landing harrasing and finishing tool.
use it to harras your lane oponent and when he is low on health finish him off.

Nether blade is not as op as his other skills but it can be helpfull for farming, manaregen and finishing of potencial

Force pulse is very nice farming and harrasing skill as well. use it to pickpocket tons of minions and harras your lane oponent whenever you can to get fed.

Riftwalk is like a permanent flash with dmg. If you have a lot of cooldown reduction you will be so mobile in teamfights, so you can jump around like a mad dog and assassinate anyone.
but i recommand you to kill enemy ad or ap carry.

your main goal in teamfight is to kill enemy adc or anyone low on health and never ever star a teamfight.

hard:

zed : dodge his skills and buy thornmail
vladimir : take some magic resist and health pot.
talon : harras, with thornmail piece of cake
swain : not sure, buy magic resist and ask for gangs
pantheon : some people picks him against, buy thornmail and voidstaff
mordekaiser : i think second hardest one, harras when no passive, gangs
leblanc : you are ****ed, nah try farm, harass and tons of magic resist
fizz : he can dodge, play safetly, harras, gang
tryndamere : not so hard, harras, gang, voidstaff
galio : farms a lot and pushes your lane, gang, voidstaff

ok these were the hard ones and the others are easy just get your core items and you will be ok against ahri, akali, anivia, annie, brand, cassiopeia, diana, elise, gragas, heimerdinger, karthus, katarina, kayle, kennen, kha´zix, lee sin, lissandra, lux, malzahar, maokai, morgana, nidalee, nunu, oriana, ryze, teemo, twisted fate, veigar, viktor, xerath, ziggs and zyra

that is for matchups and as you can see there is not much champions that can counter kassadin effectively.

I hope you will master him and dont get any noobs, feeders, instalockers, trolls, leavers, blamers, or any other kind of ****a*****es in your game.
see you on the fields of justice(c)paveljcz