The basic idea of this item-build is to create a powerful spellvamp Cassiopeia that resigns mana regeneration and resistance. Played right she will be powerful at every point during the game and will teach everybody a lesson about how strong Cassipeia can be. I often surprise people with my way of playing Cassiopeia and my team-mates even want me to play her, this is also the reason why I signed up with MobaFire and write this guide. Try this guide, get a feeling for the champion and you will have alot of fun with it :)

During the laning phase you are not the first one to engage until you got your ultimate, so tell your laningpartner to go in first.
Even if this wastes alot of mana, try to harass the enemy and keep him away from the minions, dont forget to do lasthits yourself, too. Try to always save at about 60% of your mana to always be ready for a spontanious fight.
Put your Miasma at a point, that you dont want your enemys to pass.
Put your Noxious Blast in bushes and check if someone is in by being buffed or not.
Hitting with Noxious Blast makes you faster - use this!

What you are not allowed to do:
Never hunt enemys that you cant kill with 2 or 3 twin fangs. Cassiopeia really is not a good char to escape with, so be smart and wait for your chance.
Dont use your ultimate in a 1 vs 1 when too many people miss, its your ticket back to your turret when the missing players 'suddenly' appear.

Advantages of this item-build:

No mana-problems at all.
Recover from harassment.
Higher overall endurability.
Totally (positively) changed fights.
Alot of damage (Strong in every phase of the game).
Enemys dont dare to engage against you.

All in all: A cassiopeia that you have never seen before!

No mana problems: Spellvamp makes it possible! From the point on that you got the spellvamp (lvl 8) you can get the bluebuff solo in some seconds whenever you want. You nearly lose no HP at all fighting against it. You can even solo the drake but experience has shown that your teammates will insist on helping you anyway.

My priority lies on Twin Fang, followed by Miasma and last but not least Noxious Blast. Twin Fang is her most important skill beneath her ultimate because it does alot of damage and you can spam it. Miasma is my second priority because cassiopeia is most effective when someone is standing stunned right infront of her. So why dont we make him stay longer at this place and instantly poison him with Miasma to be even more effective?! Her Q-Spell only keeps enemys on distance, it does enough damage to harrass enemys but not enough to stop them attacking me. Thats why i dont waste too much skillpoints in here. I even want to provoke a fight and then nuke them, so a distance-keeping harassment skill is not needed here too much.

Throw your poison in the center of the fight, ignite and spam your spells.
Use your ultimate if 2 - 3 or more players hunt one of your team-mates or crowd.
Engage! Their squishys will think you are an easy kill and all jump in to focus you, you will take alot of damage, stun them, slow them, flash out and and the rest will be done by your team. This really works and makes teamfights very easy to win.