Pros: lots of crowd control! with your w and ults slowing abilities, as well as your ults stun, all of which are AOE, it gets really annoying to be an enemy. lots of damage even early game, and very spammable abilities with her passive reducing mana cost, and her e's cd reset passive.

Cons: VERY squishy, especially early game. Damage items like deathcap need to be put off in favor of survivability at start of game.

Afraid of snakes? You should be!
Cassiopia, the Serpent of Death!

Why should you play cassiopia instead of your many other choices for an AP nuke? Simple...DOT! Damage over time! No greater feeling than watching that almost dead carry run away thinking he made it, only to die a few seconds later.

Skill order:Noxious Blast: Cassiopeia blasts an area with a delayed high damaging poison, dealing 75 / 115 / 155 / 195 / 235 (+90% of ability power) magic damage over 3 seconds and granting her 15% / 17.5% / 20% / 22.5% / 25% movement speed for 3 seconds if she hits a champion.

This is the ability that I max first. You don't see many Cass players doing this, but there's 3 reasons I do. 1) farming ability! 2-3 hits of this on minion waves and they are toast, allowing you to last hit at your leisure and out farm just about anybody in mid without putting yourself in danger. 2) the ms boost. getting chased? chasing? no prob! hit the champ with this and take off! 3) Twin fang....
Twin Fang: Cassiopeia deals 60 / 95 / 130 / 165 / 200 (+55% of ability power) magic damage to her target. If the target is poisoned then Twin Fang's cooldown is reduced to 0.5 seconds.
I take a point in this at lvl 2, and max it 2nd. Whats great is as long as you keep your target poisoned, you can spam this till you run out of mana or they die.
Miasma: Cassiopeia releases a growing cloud of poison that lasts for 7 seconds, giving vision of the area. Any enemy that passes through it is poisoned for 2 seconds, dealing 25 / 35 / 45 / 55 / 65 (+15% of ability power) magic damage each second and slowing them for 15% / 20% / 25% / 30% / 35%. Continual exposure renews this poison.
I take this at lvl 3, and max it last. It's great for chasing, ganking, escaping, and farming. Use it on a champion to slow them, allowing you to escape or preventing them, as well as poisoning them: remember, once poisoned your twin fang's cd is .5 sec. Noxious blast and then lay miasma down to nuke minion waves without ever having to auto attack them.
Petrifying Gaze: (Active): Cassiopeia deals 200 / 325 / 450 (+60% of ability power) magic damage to all enemies in front of her. Enemies facing her are stunned for 2 seconds, while enemies facing away are slowed by 60% for the same duration.
I love this ultimate, as it's so versitile in it's use. Early game, us it for a quick kill on an over-extending enemy. My favorite way to do this, is to bait them into chasing me out of the lane. soon as you get them onto your side of the lane, turn around and pop ult. now nuke them at your leisure.

Passive:Deadly Cadence: After casting a spell, any subsequent spellcasts will cost 10% less for 5 seconds. This ability stacks up to 5 times.
This passive make cass especially deadly, as you can spam twin fangs all you want and not have to worry too much about mana.

How do you use your skills to become public enemy #1 the rest of the game? glad you asked!
The ideal order:
R
W
Q
E
E
E
E
W
E, ect

Explanation? Easy. First, hit your ult to freeze them in place. now, hit them w/ your noxious blast and let that poison sink in. Next, lay your miasma down. This will keep them poisoned as long as they stand in it, as well as slow them down if they try to run. From here, it's as easy as spamming your twin fang, stopping to noxious blast them in order to keep the poison going, before returning to your fangs. Throw down your miasma to keep them from running away, and keep them poisoned. They'll die as they run :)

Here's a great way to use the ultimate tho. Say your team is trying to push middle, when suddenly the enemy pops out of the bushes all together in one big group. Sucks to be a carry at this point, doesn't it? Not w/ cass on your team! Throw the ult out there and freeze the entire enemy team. Throw down miasma and noxious blast, and their carries will be dead before they even get a chance to think about it!

Items:Tear comes first. As much as you spam her abilities, you'll max it out rather quickly, and mana won't be a problem afterwards.
Second I take my stage 1 boots to help me get around. Later on I'll turn this into merc treads most often for the MR and Tenacity.Deathcap comes as soon as I can build it.Crystal Septer orVoid staff come next. If the enemy is starting to stack MR, I'll take void first, if not I go septer first then the staff.Next I finish off my tear into an Archangels.
Here's a fun note about Rylai's for you. Once you have it, you have 3 aoe slows and an aoe stun. obviously your w slows, and so does your R if they have their back to you. but rylai's adds it to your q as well. EPIC!
From here your dmg build is done, and you can focus on survivability. The basics apply here. Getting torn apart by an AD champ like Tryn? Thornmail. AP Nuke giving you a hard time? Force of Nature or Banshee's veil.

For runes I go with flat AP marks and seals, magic resist glyphs (survivability), and magic pen quints. With this set-up, I'll start off w/ more AP and MP than players who start off w/ a Doran's, allowing me to skip that item.

For runes I do Magic Pen marks, and AP/AP per lvl seals and glyphs, with Magic Pen quints. Personally, I find the extra dmg to be far more useful early game than any defensive runes. flash is used almost exclusively as a means of escape.

I like to use teleport and flash. teleport is mainly so that I can leave the lane to buy, and get right back in with minimal delay, as well as the obvious benefit of being able to help other lanes.

During team fights, try to stay towards the back, hide if you can, and don't be afraid to hit and run. if you have to enter and leave a fight repeatedly don't worry about it, you're far more useful alive. Try to save your ult for when you have the majority of the enemy team in range and facing you. mass nuke and stun makes for a much easier team fight.