Pros:

+Amazing mid laner, great harassment
damage, amazing farm, high range and
movement speed.
+Has great number of disables for a
single team member.
+Very hard to gank, even due to being
squishy, slow with Miasma and hit
with Noxious Blast to gain speed,
use Petrifying Gaze and Flash if
needed.

Cons:
-Very squishy, this is a problem you
can quite easily get over with some
positioning, but until you do, you
will find Petrifying Gaze quite
hard to land.
-A lot of skill shots, meaning she
needs practice, but this is not so
much of a con for me, it's where the
fun is hidden.
-Un-reliable disables, needs skill.

Deadly Cadence(Passive): I am not sure if there could be made a better and more fitting passive for . It helps so much with mana management from early to late game, along with mana runes, you practically can't lose mana if you use your mana correctly. When low on mana and have to attack or cast to defend yourself it can be a life saver, really amazing.

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SPACE Noxious Blast (Q): The core of are her posions, and this is primary one, the most damaging potion she has, the most spammable one and it even gives her bonus movement speed, it is not hard to land and with a bit of practice you'll get a hang of it, it is just like Karthus's Lay Waste, press Q and after a short delay (about 0.5 seconds) a poison will blow up and affect targets in small AoE.
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Tips and Tricks

Use it to spam in lane constantly, do note if you stack up to 5 Deadly Cadence stacks with this and keep spamming it just when Deadly Cadence is about to run out, along MP5 runes you will lose no mana, but be careful and don't push your lane too hard.
You can hardly make strategy above work if enemy has Boots of Speed, if that case just farm and when enemy comes in for the last hit, punish him with a Noxious Blast, it always works for me.
Next to being offensive, it is much of a defensive tool too. If you are escaping use this on enemies behind, it will give you MS and damage them and just in case they are unskilled (stupid :x) enough you can eventually turn and finish them off.
When you hit an enemy champion with it, you will see buff icon on you, this can be used to very well check bushes.


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SPACE Miasma (W): Let's go over the mechanics of this skill first, so basically you throw down the cloud of poison and it instantly slows and starts spreading to apply the same effect, it also gives vision. This is mainly your slowing and farming skill, while spreading circle is quite useful in team fights.
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Tips and Tricks

At level 5, much like Morgana you can use this skill in combination with Noxious Blast to clear whole ranged mob way in the back, amazing farming tool and I use this strategy all the time, while being careful of how pushed the lane is. Very effective against Malzahar and Brand.
If you have the blue buff, you can use this skill again before it's duration goes away, use this advantage and mana regeneration to scout bushes, never head in and face check, that is pretty much a death wish.
It is much easier to land than Noxious Blast so you might want to land it first and slow the target before hitting off your Q.




Twin Fang (E): So how this skill works, you hit someone with your poison and then you use this skill on them and it's cooldown is refreshed to 0.5 seconds instead of 4. AD carry has auto attack, you have this, no better way to put it, of course don't use it to farm.
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Tips and Tricks

One of most often made mistakes, even by me would be hitting Twin Fang before proccing your Noxious Blast, and yes this can happen, due to Noxious Blast having delay on it, you might wanna wait for it to hit before instantly firing Twin Fang, you might end up on 4 seconds cooldown, which is not nice. ESPECIALLY watch this after you Petrifying Gaze someone, make sure your poison procs first.
Yes, this skill works with other poisons, not just your own, but don't rely on this. It is just a safety net for you, still aim your poisons because DoT deals amazing damage.
In early game while this is level 1, you don't really wanna spam it on enemies, use only Noxious Blast to harass, don't follow up with this UNLESS you are going in for kill, it will burn your mana down and distract you from much needed farm.
One thing I discovered is that, this skill eats Morgana's Black Shield alive, just spam it on her shield and it will be gone before you know it, laning against Morgana is not really a problem.
You can always smart cast this skill, even thorough I don't do it, it can be useful it really depends if you prefer it so or not.




Petrifying Gaze (R): Ah, finally, my favorite thing about Cassiopeia, her ultimate. This amazing skill is high damage stun/slow with a "short" range. Actually range is not short at all, it is all question of positioning, if target is facing you he will be stunned, if not he will get a decent slow, both of these allowing you to easily follow up with your skills.
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Tips and Tricks

Think about position, think outside of box, no one said you are limited to being in front of target or close to them, this skill is actually most effective when used over walls or trees and you can see that in video below.
With Rylai's Crystal Scepter the slow will be very high and it might compensate for missed stuns, but remember stuns are better.
Try to use it from the bush, enemies won't see it coming and you have the highest chance of hitting the stun.
It is very effective for escaping, use it and then use Miasma and Noxious Blast on stunned target, you will be out in no time.
I love using it for mind games, when some cocky squishy (mostly Vayne,or Ashe) is chasing me, I just turn around, stun her and then burst her down with Miasma Noxious Blast Ignite Twin Fang combo.

Early Game:

So, you had picked Cassiopeia in pick, make sure no one else goes mid, now I will explain what you do from top to the bottom so let's begin.

You are in loading screen, take a look at other champions and their summoners, you will almost -always- see CV. Once you enter the game wait about 20 secs before doing anything, make sure you don't get CV to reveal your items. This is important because Boots of Speed give powerful start if not expected and a lot of AP mids tend to start with Doran's Ring.

Now that you are in your lane, make sure to last hit your minions, the thing to remember is that you are harassment queen and no one can stand up to you there, so whenever they are in range remind them of your presence with a Noxious Blast, don't let them last hit, harass the enemy.

If enemy tries to harass back, your boots will let you avoid skill shots and you have 3 Health Potion so you should have no problem.

Don't use your skills to push, use them to harass or push back. Pushing back means if you have Brand or Malzahar against yourself you can use your skills to push back the minions since these two push quite hard. Request for a gank from jungler and try to score that first kill, if they kill you first, no big deal you will get back in there and continue fighting.

At level 5 or 6 you will have around 1000 gold, go back buy 2x Doran's Ring 2x Sight Ward and 2x Health Potion. Put a ward in ONE of the side bushes and stick to that bush, this will give you great escaping curve and you will practically be ungankable unless they have Shaco or Rammus in which case I suggest warding both bushes.

Keep on playing like this, try to score kills, farm and deny your enemy, until you are able to either go gank side lane or destroy the tower, this is what you early game is consisted of, more on specific laning opponents later.

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Blue buff:
Blue Buff Initial Spawn: 1:55
Blue Buff respawns in 5 minutes
Total XP: 280
Total Gold: 80

Being Cassiopeia you are naturally mana hungry and a spammer, blue buff here helps you a lot, but don't expect to get the first blue buff, you will hopefully get the second one. Once you have the blue buff start being a lot more aggressive, spam without stopping and harass enemy's mind out. Of course respect line between aggressive and reckless, don't overextend while harassing. If your jungler is Fiddlesticks or Nocturne make a sad face and forget about this buff completely..

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Mid Game:

You love this part of the game, you got your Twin Fang stronger and you can do some serious single target and AoE damage, mid and late game are pretty much the same, try to gather your team in one lane and push out the first set of turrets if you didn't already.

The most important part of mid game for you is positioning, you must work and practice on this, no guide can help you, but remember your spells are not restricted by walls, not even your ulti, do feel free to take a safe spot and spam your abilities.

Another good strategy is to get into bush and cast from there, always be behind a tank and don't stupidly rush in like I do sometimes, yes I get cocky...

Remember your goal is victory not an amazing K/D/A, so head towards pushes and always have a ward on dragon, you really wanna control dragon and take it every single time you can, also ward Baron Nashor because if enemies get it, it can be nasty. Due to your naturally long range you can harass enemies who pile up to try and steal him, but be careful and don't do this if there's Blitzcrank on the enemy team.

Rest of the game should be push for victory.
Team Fights(Late game):

This was requested part so here I am writing it up, how should Cassiopeia act in late game team fights.

First of all the biggest mistake by most Cassiopeia players is rushing in close range to get that Petrifying Gaze off. This may be alright if you have Zhonya's Hourglass and can do it properly while placing Miasma and Noxious Blast after and not getting stun on your head. It is really hard to do so I don't exactly recommend it.

What I do recommend is standing back on quite a range, let your tank and melee off-tank DPS initiate the fight, you should come in a bit later. Your range is very high on Miasma and Noxious Blast, use these two to attack the squishies, or well the target team is focused onto.

A bit later when their protection is gone, which you should judge, you should come closer and spam Twin Fang to finish them off. As for your ulti, it needs special mention. I suggest you to position yourself to try and use it over wall, or maybe you can flash into the fight suddenly and use it, but make sure your tank is always in front of you, chose the moment when you can catch most people into it.

I don't suggest you to look for moment when you can stun all 5 because you will never use it, use your ulti on most people possible for DMG + Slow and if someone is stunned then you are at luck - of course this only counts for team fights, in 1v1 or 1v2 or so, always try to make stun priority over DMG.