Hello!
Im Dared. I really like Kassadin and i hope that you will enjoy that guide and hopefully learn something from it. *sorry if my English is bad*
:)
The guide is for custom play
I will show you the most needed items/masteries/runes and etc, and you will decide with wich one you are gonna play.

If you never played him - you will know the basics
If you played him - you may learn something usefull

Masteries: I go with the basic 9/0/21.
1. I dont take the Greed mastery. 180 gold for a 30 minute game isnt worth it. :C
2. Expanded Mind > Awareness. Yes, i know. 5% XP is good, but i will need 5% mana instead, because i usually get oom when i spam, and ^Mana gives ability power^. You will find out why in the Items chapter.
3. Burning embers - This is a talent for improved ignite. , but its just not worth it. You will loose 15% Summoner Spell cooldowns, or 15% magic pen, not worth it for 10 ability powar.

Runes:
*Flat mana regen - you need some mana regen, especially in the Twisted Treeline, because there is no blue mana buff.
*Ability power - I FEEL THE POWAHHHHHHHHH!!!
*Magic pen - With boots for m.pen/talents/and some runes - maxed vs glassy champions
*Cooldown reduction - Thats very good for Kassadin, especially for the Q spell...and the W spell...And the R spell...oooo and the E spell somethimes...You just need that :|
*Flat health - May be usefull at the beginning.
Some runes that i dont use but some people find them usefull:
*Flat mana - You will have enought mana at the beginning, but it wouldnt change too much.
*Mana per level - I think that its good if you have Archangel

* Attackspeed - Well for me - this is a no. I respect other guide's opinions, that attackspeed is usefull for Laning, PVP because of the W spell, But if you want more damage from the W spell - Ability power. If you want more mana from it - Mana regen instead.

^Mana gives Power^ <--- [*] Archangel's staff. 3% of your mana is ability power, aint bad.

Now the items that i use/find usefull.

1. [*] Rod of ages - Very much health and mana, and ability power, also helps you laning with the heal when you level up. Its a need.2. [*] Archangel's staff - NEED! 1400 mana when maxed (meh), mana regen, ability power and like i said, 3% of your mana is POWAH!!!! its a need.3. [*] Mejai's Soulstealer - Ability
4. Boots...ofcourse :D I take the Cooldown red ones, but [*] Mercury's treads and [*] Sorcerer's boots are options too. 5. [*] Frozen hearth - Mana (powah) alot of cooldown red, 100 armor and slows the attackspeed of nearby enemies. This is one of the most important items vs a DPS team. 6. Speaking of counter dps - [*] Thornmail!!!! i dont know why, but alot of people forget/dont know about this item. 100 armor and returns 30% of the physical damage taken.7. [*] Lich bane - Yes, mana, ability, resist, movement and the best passive in the game (for mages)8. [*] Rylai's Crystal Scepter - 500 hp, ability and slow - worth it.9. [*] Abyssal scepter - You know you want it :p10. [*] Void staff - ability and M.pen (40%) worth it.11. [*] Will of the Ancients - Ability power and Spell Vampirism aura - FTW~
12. [*] Rabadon's Deathcap - Its worth getting ONLY for the 30% ability increase, and 155 ability power bonus - Must...have...13 [*] Nashor's tooth - Not bad especially for hybrid buidls.14. [*] Morello's evil tome - not bad.
15. [*] Zhonya's Hourglass - 50 armor 100 powah and 2 seconds godlike....AInt bad man :|


!!! It isnt bad to get more then one Rod of ages or Archangel staff !!!

Ok you can choose between
R > Q > E > W anti mage
or
R > E > Q > W spell tacking
*remember* if you want to cap your Force Pulse (E) you will need to have atleast one rank of Nether Blade (W) before level 6. It willgather you mana and will get you stacks for Force Pulse (E)

or R > W > E > Q thats for hybrid builds with items like Guinso or Nashor tooth.




Those are the spells *if you really dont know*
Null Sphere - Kassadin fires an ethereal bolt of void energy, dealing damage and silencing the target for a duration.
Kassadin fires a void bolt dealing 80/130/180/230/280 (+0.7) magic damage and silencing the target for 1/1.4/1.8/2.2/2.6 seconds.
Cost70/80/90/100/110 ManaRange700


http://www.leagueoflegends.com/sites/default/files/game_data/1.0.0.118/content/spell/38-2.gif Nether Blade - Passive: Kassadin's melee attacks restore mana. Active: Kassadin gains bonus armor penetration for a short duration.
Passive: Kassadin's melee attacks draw energy from the void, restoring 4/8/12/16/20 mana each hit. If this effect is triggered from attacking a champion the amount of mana restored is tripled.

Active: Kassadin charges his nether blade, gaining 7/15/25/38/50 armor penetration for 5 seconds.
Cost25/25/25/25/25 ManaRange1


http://www.leagueoflegends.com/sites/default/files/game_data/1.0.0.118/content/spell/38-3.gif Force Pulse - Kassadin draws energy from spells cast in his vicinity. Upon charging up, Kassadin can use Force Pulse to damage and slow enemies in a cone in front of him.
Kassadin draws energy from spells cast in his vicinity, gaining a charge whenever a spell is cast near him. Upon reaching 6 charges, Kassadin can use Force Pulse to deal 60/115/170/225/280 (+0.7) magic damage and slow enemies by 30/35/40/45/50% in a cone in front of him for 3 seconds.
Cost80/80/80/80/80 ManaRange400


http://www.leagueoflegends.com/sites/default/files/game_data/1.0.0.118/content/spell/38-4.gif Riftwalk - Kassadin teleports to a nearby location dealing damage to nearby enemy units. Additionally, multiple Riftwalks in a short period of time cause them to cost additional mana and deal additional damage.
Kassadin teleports to a nearby location dealing 60/90/120 (+0.5) magic damage to surrounding enemy units. Additionally, each subsequent Riftwalk in the next 7 seconds costs 100/100/100 additional mana and deals 60/90/120 additional damage.
Cost100/100/100 ManaRange10000

http://www.leagueoflegends.com/sites/default/files/game_data/1.0.0.118/content/spell/38-5.gif *Passive* Void Stone - Kassadin takes 15% reduced magic damage and transforms this damage into bonus attack speed.

Usefull:

*Ignite - no need to explain
*Flash - mmm.. 2>1 right?
*Cleanse + Riftwalk = free escape/chase
*teleport is for ganking/laning
*CLV - sometimes it will work better then expected, but supports should go with it
*Heal - Saving your life
*Clarity - Saving your..mm..mana

Not usefull:

*Running is too mainstream for ghost...Teleportation is better :p
*Fortify - no lol
*rally - no loool
*revive - NO LOOOOOOOL
*Smite - No #$#!#!@#$!%

Teamwork: Gank the other team
Help your team
Join meles

Teamfights: Ok in teamfights you need to do: Fight starts, you wait in the brush/behind a wall, then you teleport, kill someone with your combo, and flee again.
Some may call you kill stealer, but when you get your items - you will make their game.

Combo: Riftwalk in, QWEQWEWQEQWEQWE, Riftwalk out.
Make sure that you have the blue/red buffs too!

Blue buff - You need mana, with it - you aint gonna get oom.
Red buff - You dont rly need this, well you do when you start hackin/slashin with your nether blade, but its worth it ONLY for the cooldown reduction.


!!! Killsteal as much as possible to win !!!

Good:
teleportation
heavy brust damage
Anti mage
Slow,silence,teleport,asdasd,ragequit
Looks awesome (half vader half nakid half finnish LOL)
You can escape everything

Bad:
Glassy before you get Rod of Ages or Crystal scepter
Kinda low before level 6

Farm the Blue/red buffs and the dragon.