Cassiopeia is one of the strongest AP Carrys at the moment. She's able to harass and take down several opponents at the same time with her abilities. Cassiopeia is an extremely good farmer, With her Q and W she's able to farm as hell because of the AoE.

Will Of The Ancients is an important item on Cassiopeia, because the Spell Vamp let's you too easily get your health back with the help of your AoE Abilites.

21/0/9 Masteries does also work on Cassiopeia

Noxious Blast
This is an AoE Ability that is easily to farm with, It has a small delay before it explodes and it makes your opponents getting poisoned.

Miasma
Miasma is also an AoE Ability, It makes a puddle on the ground that makes enemies getting poisoned when they are walking on it. They does also get slowed.

Twin Fang
Twing Fang is Cassiopeia's main ability, Twing fang is the ability who makes all the damage, If u cast Q or W on a enemy they get poisoned. Cassiopeia are then able to cast Twing Fang up to 4 times on a enemy while they are poisoned.

Petrifying Gaze
Petrifying Gaze is Cassiopeia's ulti. If the enemy stands in the front of you and look at you, they take both damage and get's stunned. If the enemy looks in another way and not into your eyes, they take damage but doesn't get stunned.