I'll start with a little introduction to cassiopiea.

Cassiopiea is a fairly underestimated AP carry. She's very unique in the way that she relies heavily on sustaining damage, NOT nuking. Although some say Cassiopeia can be built to nuke, i still really dont see how she can.

Cassiopeia's skills rely heavily on DOT damage (Damage Over Time, for all you newer players out there

Speaking of her skills let me give you a little information of each of them. I will go over them more in-depthly in another section.
Deadly Cadenence - Cassieopiea's Passive. Each spell cast within a five second time period costs 10% less mana (effect recharges with each spell)
Noxious Blast - Cassiopiea Blasts an area with a high damaging poison (small delay before exploding)
Miasma - Cassiopiea infects an area with a growing cloud of posion that slows and poisons any enemy to walk threw it, poison IS renewed if target walks through again or stays in the area. (cloud will grow for 7 seconds.)
Twin Fang - Cassiopiea shoots a fang dealing magic damage to a target. if the target is poisoned this ability's cooldown is reduced to 0.5 Seconds.
Petrifying Gaze - Cassiopiea Deals high magic damage to all enemies in front of her, if the targets are facing her they are stunned for two seconds, if they are looking away they are slowed by 60% for the same duration.

I hope you guys do Enjoy the guide, this is my first guide so please be patient with me, and leave all constructive critism you can! I hope you like it!

Easily the Best Summoner Spell in the game, Gives you an ALMOST instant escape, or can very easily allow you to catch up to someone. TAKE THIS.
Can be replaced with Ignite if it is your preference, but i like this, because like flash is an excellent escape and chase mechanism, and I find with Cassiopiea ignite isn't required when she basically has a built in ignite, just doesn't do true damage.

AGAIN. Summoner spells are to be picked to your preference. if you wanna lane rally and smite, by all means please do. i'm just telling you my preferences.

Pro's

Very Powerful Harass.
Fantastic Solo-Laner
Faster than some other AP champs.
Very Strong throughout the entire game.

Con's

Like Every AP caster, she's very squishy.
Often Focused in team fights.
Not much Nuke Pontential
You'll be underestimated by your team mates.
Difficult to Master. But with High reward.

EARLY GAME (1-9)
Focus on Farming as much as you can unless you are against someone you can harass easily.
Dont use your abilities to harass too much until around level three, when you can start harassing harder. If you really want to harass your opponent out of lane, ask your jungler for blue buff. Cassiopiea has a weak early game, but not as weak as other AP champs.

MID GAME (9-15)
Cassiopieas time to shine. By now you should have a decent farm if you farm well. Your team and yourself should have nice warding, and your ready to start killing and ganking (assuming you haven't done so yet.) You should be putting out heavy harass on your lane opponent and if you knock your lane tower down. stop pushing. only come back to protect your lane, you dont want to over extend and shamefully die. start ganking and helping other lanes that are having trouble, in effect be a second jungler. (just dont literally jungle of course)

LATE GAME (15-18)
By now you should be a killing machine. if your smart every enemy that crosses paths with you should be dead. You should focus on moving together with your team and making smart engagements. Allow your tank to run in first and position your self behind the fight, spam noxious blast and twin fang, let the fight brew a bit before tossing in your miasma. If they decide to chase you or rush at your team, punish them for getting close and ult. this should be enough to win you the fight.

Deadly Cadenence - Use this passive to spam away at abilities (dont be stupid and waste all your mana because this doesnt solve your early game mana problems AT ALL. this is a very good passive for the mid game laning phase, because you can start harassing much more powerfully.

Noxious Blast - Cassiopiea's bread and butter. works like Karthus's "Lay Waste" Cassiopiea throws a small poisonous bomb that explodes after a delay about half a second. poisons the target and if it hits an enemy champion give a nice speed boost for a short time. use in conjuction with your Twin Fang to harass.

Miasma - Throws a small cloud of AoE Poison that grows larger over 7 seconds. slows anyone who walks through it, use this for escaping (combine it with Noxious Blast for even better chanes) also use during a team fight to poison the entire team and spam your twin fang alot.

Twin Fang - Simple. Don't use this unless a target is poisoned or very close to dying. Use in conjunction with your noxious blast to harass or effectively kill your opponent. nothing much else can be said about this.

Petrifying Gaze - One of the Best Ults in the gme. can effectively stun an entire team. in a team fight run up close and use it when people target you and sttun then let your team do the rest.

Well I do hope you guys enjoyed my first guide. Cassiopiea is a very powerful champion, who is extremely underestimated.If played correctly you will DOMINATE your games. please leave any constructive criticism you can, I'd appreciate it, and please dont rag the guide before trying it, be sure to come back and upvote if you like it! Add me if you'd like, my SN is RoG Skorpiion (yes two I's)

-Skorpiion.