Cassiopeia is a mage. A poison mage who specializes in getting as much damage out as fast as possible. And this can be done by spamming every poison skill at once.

15/12/3 is what seems to work out. I can go into a fight, stay back, and still get damage off with these.

In order, your items will be like this:
Rod of Ages
Rylai's Crystal Scepter
Deathfire Grasp
Sorcerer's Shoes
Hextech Gunblade
Haunting Guise
Rabadon's Deathcap

Flash and Ignite. Ignite is good at early levels because when your 'Q' skill is poisoned, it will take the enemy down to lower health. Ignite will do the finishing blow once they go to Turret Hug and recall. :3

All in all, this guide is a KS-er's guide for Cassiopeia. Thanks for reading!