So this is my first build posted. And this is how i prefer to play Cassiopeia. This build provides great sustainability in lane, solid escape in the event of ganks, and overall speedyness making you a tough opponent to catch between your Noxious Blast and Miasma.

Simple Breakdown of runes.
(red)Magic Pen = More Dmg

(yellow)Mana regen/lvl = This will help you all throughout the game as my build features a tiny manapool.

(blue)AP/lvl = So you hurt more.

(Quint) Flat AP = More pain early on.

Masteries run 9-0-21 Standard caster masteries, though i personally like to take increased buff durations at 2. I do this because, unless there is a jungler on my team. I can easily push a lane, grab blue buff, and be back before the minions are pushing my tower.

So with this, since Twin Fang is your main source of damage. You max this first. I like to max miasma second for the increased slow, Though this is not a very high damage ability.
And lastly noxious blast. I level this last, as its the most easily spammable spell, and keeping this low level for the low mana cost allows you to keep you passive up for your more mana hungry spells. Not to mention it boasts a decent AP ratio for such a spammable spell.

Again, as my build has little in the way of mana pool, Clarity becomes useful all game. For most champs i find it falls off at lvl 6+, on Cassio, this spell will shine all game with my setup.

You can compliment this with anything you would like, I would recommended ghost or flash.

Ghost = with boots 5, insane range, and you movespeed buff from your Q. You will be extremely hard to catch.

Flash = Great for getting that perfect alignment with your ult, also a great escape mech. Great pick for any champ always.

Ignite = More Dots + Reduced heal. Good pick for Cassio to be certain that champs like vlad or mundo will be hard pressed to keep their lifesteal/regen up against ur dps.(considering your not a burst mage)

Exhaust = Meh. I dont like it on Cassio, you already have a built in slow, a built in move buff. And with these 2 tools you should never have to get in range of you enemy untill you can make the killing blow.

Clairvoyance = Let a support get it.

Fortify = Let a support take it.

Revive/Rally= No

Key thing to keep in mind. Clarity is a MUST with this build. After that Ghost/Flash/Ignite.

Most importantly, Items and why.

Dorans Ring - 15 ap so you hurt more. Mana regen for the Lack of mana regen early game.

Boots of Mobility - Considering your harassment abilities. You should be able to maintain a range where you wont be hit. Keeping the +5 move speed. Generally i take these because you can easily drop a miasma/noxious blast and push your lane. Slither over to blue buff. And be back before the lanes in your end again. Or go support a lane in trouble with ease.

Hextech revolver - This will give you insane survivability all throughout the game. As well as allow you to take Blue buff while taking no damage. If you get low, then drop a miasma and noxious blast on some minions. Your hp will come back pretty quick.

Rylais Crystal Scepter - Slow on your Q. Increased slow on your W, More slow on your E. This will make it way easier to land your combo as slower enemies = easier hits for you.

Deathcap = This should speak for itself.

WoTa = Bout time to upgrade that revolver into a Wota. Because more spellvamp/AP + an aura is just awesome.

Void Staff = This is your only source of Mpen after runes. A must.

Sell your dorans and get whatever you need. I like Zhonya's hourglass or banshees viel.

This is my first build and I am open to any constructive criticism or feedback.