Kassadin is a great AP burst champion who has great mobility.
He is one of the best chasers, who has a spammable "flash" ability.
Although he may be squishy, he is a great threat to the enemy.

Kassadin's passive ability ( Void Stone) allows him to be a great champion against other AP casters.

- Provides Magic Pen allowing Kassadin to do more damage to champions with Magic Resists.

- Provides a bit of mana regen. Kassadin will run out of mana quickly, so this will help.

- Provides ability power allowing him to do more damage.

- Same as above

21/0/9 Masteries for high AP damage

Upgrading AP masteries/Damage dealt on Offence, and also upgrading Ignite mastery.

Upgrading Mana, mana regen, flash CDR, Movement Speed

Flash adds another "flash" ability to Kassadin. Double flash can quickly get you in and out of dives/fights/etc.
Ignite is for the extra burst damage, and to kill off enemies with low HP.

]Null Sphere (Q) - This is his primary damage ability and should be maxed first. Silences enemy when they are hit. You can use this to help you farm/harass during the laning phase.

Force Pulse (E) - You secondary damage ability since you will need 6 charges to use this ability. It slows enemies you hit that are in the cone. This can also help you farm.

Nether Blade (W) - Does not provide the burst damage, so get this ability at the very end. But if you are having serious mana problems, then getting 1 point at level 4 can help your early game. Its active provides Kassadin with Magic Damage for melee attacks. Upon hitting minions/champions, Kassadin will regenerate a small amount of mana. Later in the game, this ability can help you reach 6 charges for your . As Symphunny informed me, the Bonus Attack Damage you receive from this skill can affect Inhibiters/Nexus, but not the towers.

Riftwalk (R) - This is Kassadin's ultimate, which provides great mobility. He damages units in a small cirle as he "flashes" to the area. This ability can be used over and over again to be stacked. However, each consecutive time you use this ability, it will cost more mana, but do a lot more damage.

Core Items

Good Items

Void Staff - Gives Magic Pen so that you can inflict more damage, even through magic resist items.
Zhonya's Hourglass - Provides a cool active, along with AP, which may sometimes save you during team fights. Survivability against AD Champions.
Abyssal Scepter - Provides AP and magic resists if you are having problems against an enemy caster.
Banshee's Veil - A defensive item for magic resist with a spell shield bubble.
Archangel's Staff - Provides the Mana regen Kassadin will need. Unless you are getting the blue buff most of the time, you might need this. Kassadin runs out of Mana quickly since you will be spamming your spells.

Other Items

Lich Bane - Not the best on Kassadin, but provides a Melee attack boost after you use a spell.
Mejai's Soulstealer - A very situational item that you would only get when you know you will be fed hard.
Moonflair Spellblade - If you are having problems with CC, this tenacity item will help you a little bit.
Morello's Evil Tome - Provides AP, mana regen, CdR.
Quicksilver Sash - An item that acts like Cleanse, this item will help you greatly against CC, but if you buy this, you won't have as much burst damage/HP that you might benefit from other items. I would rather get Banshee's Veil over this since Kassadin can escape very well.

Wait until you get 6 charges for Force Pulse

-> -> -> -> ->

If you need to escape quickly (can't wait for your Rift Walk Cool down) you can out.

If you want to do more damage with your Ultimate ( Riftwalk), charge it up before going in.
CAUTION, you will use more mana when you charge your ultimate up.

Do NOT initiate a team fight, that job is reserved for the tanks.
You are an assassin. All you do as a Kassadin is to burst down the Squishy's/Carries. You can easily "flash/ult" to them and bring forth terror.
Silence , with , anyone who has a strong AoE Spell, such as , , , etc.