Kassadin

Middle

Try that Again

Reliable and strong lane harass with strong ramping teamfight/all-in damage.

Sorcery

Empowered abilities and resource manipulation

Arcane Comet

Hitting a champion with spells hurls a comet that deals adaptive damage at them. Landing more spells reduces Arcane Comet's cooldown.
The Ultimate Hat

Gain cooldown reduction and max cooldown reduction on your ultimate. This is increased further each time you use your ultimate, up to a maximum.
Celerity

Gain 3% extra MS. Gain extra AP or AD, based on your bonus MS, Adaptive.
Scorch

Your first ability hit every 20s burns champions.

Precision

9% attack speed, 10 ability power or 6 attack damage

Presence of Mind

Takedowns restore 20% of your maximum mana and refund 10% of your ultimate's cooldown.
Coup De Grace

Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.

WhyArcane Comet

Kassadin's strong slows on [E] makes Comet incredibly reliable to land for that extra lane harass damage.

WhyThe Ultimate Hat

Thanks to the already short cooldown on Kassadin's [R], he is able to get full Ultimate Hat stacks very quickly. This lets you take a few extra Riftwalks in the mid game for chasing kills/escaping, where you other wise not be able to.

WhyCelerity

Celerity on Kassadin allows you to keep up with and stick to your opponent when you're running them down in an all-in.

WhyScorch

Scorch is a rune that, like Aery, is very strong and reliable during lane. These two combined make for a strong poking tool with [Q].

If you're looking to scale into a longer game rather than harass in lane, Gathering Storm is a decent option here.

WhyPresence of Mind

Presence of Mind is a fantastic rune choice on Kassadin as it allows you to repeatedly [R] after leveling up or getting a kill/assist in teamfights, building up and ramping your damage incredibly.

WhyCoup De Grace

Coup De Grace is the most reliable damage amp rune for Kassasdin, especially when you're able to stick to the back-line enemies and blow them up with [R].