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.
Manaflow Band

Hitting enemy champions with a spell grants maximum mana.

After reaching 250 mana, Manaflow Band restores 1% of your missing mana every 5 seconds.
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.

Resolve

+65 health and +10 AP or +6 AD, adaptive

Bone Plating

After taking damage from an enemy champion, the next 3 spells or attacks you receive deal less damage.
Second Wind

Restore health over time based on your missing health.

WhyArcane Comet

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

WhyManaflow Band

Manaflow Band empowers your ability to harass in the early game laning phase with constant [Q] poke. The additional mana pool in the mid-game should grant you a few more [R] casts.

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.

WhyBone Plating

Bone Plating's early game mitigation is fantastic for absorbing damage and helping you get through your weak early game so that you're able to scaling into a strong mid-game and take over.

WhySecond Wind

Second Wind's reliable health sustain further empowers your ability to sustain in tough lane match-ups so that you're able to scale into the mid-game.