Cassiopeia is one of my favorite champs to play with and is hard to master(not saying that I have). This build is very different from others you will see in that I don't focus use any much cool-down reduction but focus on making her very hard to kill early game and dangerous at end.

PROS:

Good Harasser with her Q
Her Ult can turn the battle in your favor or save your ally's ***
Can Farm very well
Good Spammer
Katarina's sister

CONS:

Not easy to play
Squishy
Her Ult requires good really good timing and has wide radius but low range
Ult is a little buggy

9 Greater Mark of Magic Penetration- To make her spells hit harder
9 Greater Seal of Replenishment- For improved mana regen during early/mid game so you can lane longer and spam spells
9 Greater Glyph of Scaling Ability Power- good amount of ap every level
3 Greater Quintessence of Health-Cassiopeia is very squishy these will help a lot early game.
OR
3 Greater Quintessence of Ability Power- Early on your spells will do a lot of damage especially your Q.

10/0/20 Cass doesn't need CDR so I instead get:
10 in Offense for exhaust buff,ap,and magic penetration
20 in Utility for mana regen,increased mana,exp,flash cooldown reduction,longer blue buff, and the health regen + spell vamp early on will help you stay in lane longer and heal.

Passive: Deadly Cadence

After casting a spell any subsequent spellcasts will cost 10% less for 5 seconds. This ability stacks up to 5 times.

Pretty much spamming your spells will make them cost less up to 50% less

Noxious Blast

Cassiopeia blasts an area with a delayed high damaging poison, dealing 75/115/155/195/235 (+0.9) magic damage over 3 seconds and granting her 15/17.5/20/22.5/25% Movement Speed for 3 seconds if she hits a champion.

Cost
35/45/55/65/75 Mana
Cooldown: 3 seconds
Range: 850

Get this first. It explodes near your mouse cursor and does a certain amount of damage over 3 secs. For example if it did 30 damage it would do 10 damage every second for 3 seconds. I like to use Miasma first so they are slowed making giving this a higher chance to hit them

Q Combos
Q-W-E Basic Combo-I use when harassing
Q-W-E-E-Q/Q-W-E-Q-E Advanced Combo- Most of time these are the combos i use the poison is re-applied making sure your E always refreshes even when W disappears.



Miasma***

Cassiopeia releases a growing cloud of poison that lasts for 7 seconds. Any enemy that passes through it is poisoned for 2 seconds, dealing 25/35/45/55/65 (+0.15) magic damage each second and slowing them by 15/20/25/30/35%. Continual exposure renews this poison.

Cost
70/80/90/100/110 Mana
Cooldown: 9 seconds
Range to Center of AoE: 850
Radius of AoE: 100-175

Get at 1 point early on at least to help farm good. Also when attacking start with this to slow their escape. Most people ignore the poison but don't realize the slow. Also remember the cloud will grow. I like to use near an enemy so it will grow under them a lot of the time they don't notice that its getting closer.

W Combos
W-E / W-Q-E / W-E-Q Basic Combos- Good for Harassing
W-Q-E-E-W / W-E-Q-E-E Advanced Combos- When you want to ensure they do not get away



Twin Fang***

Cassiopeia deals 60/95/130/165/200 (+0.55) magic damage to her target. If the target is poisoned then Twin Fang's cooldown is reduced to .5 seconds.

Cost
50/60/70/80/90 Mana
Cooldown: 5 seconds
Range: 700

This is your nuke. Use this on poisoned opponents It is almost instantly refreshed if it hits poisoned enemies.The effect works with other poisons i.e. Teemo

E Combos
Only time you start with E is when they have low enough health for one shot or they already have a poison effect from someone else on them like Teemo or Twitch.



Petrifying Gaze

Cassiopeia deals 150/275/400 (+0.6) magic damage to all enemies in front of her. Enemies facing her are stunned for 2 seconds while enemies facing away are slowed by 60/60/60%.

Cost
120/160/200 Mana
Range: 850
Cone Width: 90 degrees (shaped kinda like a V)

Use this in team fights or to start ganks. This is hard to use since its only really effective when they are facing you. What I do is flank them during a gank and get in front of where they will run so they go straight toward me then I stun.

***This spell I changed it to a smart cast.Meaning I use it like Noxious Blast and its placed near the mouse cursor and no need to activate then aim. I suggest you smart cast this because then you can use quickly and easily while fleeing or chasing. Miasma benefits from this when you need to slow them down while chasing you can quickly throw it at their feet while they are running and sometimes you'll hit them activating the slow/poison. Twin Fang benefits from this because you can pull off your poison-fang combo quicker. Twin Fang auto targets when you hover mouse cursor over enemy.So you don't have a chance to miss just a chance to hit minion or another enemy if they are really close.


Escaping
If your Flash or Exhaust is on cooldown and your being chased use Noxious Blast for the speed boost and Miasma to slow. Once you get Rylai's Crystal Scepter enemies are slowed to a crawl when you use both skills

What I use

Flash- Good to use get in position for your ult or escape
Exhaust- Good when you miss with or want to stack Miasma. Mastery adds MR debuff making your poison do more damage. Can both be used to escape or get in front for Petrifying Gaze's stun.

Alternative Options

Ignite- When your poison isn't enough put this on to finish them. I don't usually get this as I don't think its too helpful later on and can't be used to escape and help kill
Ghost- Alternative for flash great for chasing and fleeing. I personally prefer Flash on Cass.

Mejai's Soulstealer - This is a core item because most of Cassiopeia's skills are AoE making easy for you to gain stacks even if you don't the kill. Early on this will make you extremely powerful your Noxious Blast will large amounts of damage early on because most won't have hp or mr items by the time you have you Mejai's and some stacks.

- I get treads instead of Sorcerer's Shoes because Cassiopeia gets focused a lot. The CC reduction is vital for you in team fights so you can get away or move out of range and throw you poisons down. The magic resist will help a lot too because your very squishy.

- Catalysts passive and Rod of Ages Passive will help a lot in battles and when laning and low on hp or mp. Since your squishy the health is really helpful. The increased mana will help in lane when you'll have usually have low mana problems.

- The health will help with your squishiness. The slow is really good for chasing( Twin Fang only slows 15%). If I'm losing I try get this as soon as i can
- When I'm winning I build this immediately so I can do huge amounts of damage. The passive increases ap gained from stacks of Mejai's Soulstealer.
- If throw a basic attack between Twin Fangs you can do a lot of damage. Good when they are building Magic Resist because your next attack will do physical damage. The movement speed and magic resist is also very helpful.

Early Game

Amplifying Tome +1 Health Potion Start with this so you can do a lot of damage with your q early on. *Tip: I try to have at least 2 health potions and a mana potion on me at any time especially during early/laning part of game they come in handy Next get Boots. Then start building your Catalyst The Protector which will help you stay in lane longer. It's passive really comes in handy when low on health or mana in a lane.Now buy a Mercury's Treads. These will help when you need to escape. Now finish your Rod of Ages.Get a point in Miasma early so you can farm/push creep waves. Use when your not sure where in brush they are but don't want any surprises. Also use Noxious Blast if it activates then you know where they are. Remember to keep moving around to last hit and be unpredictable while harassing.

Mid Game

If I'm winning i buy a Rabadon's Deathcap then a Rylai's Crystal Scepter. I get a scepter first if I'm losing usually Now you'll be ganking try to position yourself for Petrifying Gaze and keep moving around. Don't sit still too long and try to keep distance from your target so your not focused.

Late Game

2 Good items to build now: Banshee's Veil and a Zhonya's Hourglass. Banshee's Veil's passive will help when trying to escape and the extra health/mana,and magic resist will too. Zhonya's for the armor,ap and its active will help a lot. You probably won't finish the build before the end but if you do remember to use Zhonya's active and stay in the back in team fights and move around so your not focused. Also if you can get your ult off in a team fight and can stun a few people throw Miasma and move back so they don't immediately attack you and you can throw your Twin Fangs.

- Usually eliminates early mana problems once you get tear. Since you use spells quickly you can max out pretty fast
- If you need more magic penetration
- I buy if their are tanks with high mr. Works better against people with a lot of magic resist.
- I buy when I need more magic resist. The magic reduction also helps you and your team
- The active comes in handy when I am getting focused in a team fight. The increased armor is very good too.
- I get when we are losing and I need more health and the other team has a lot of CC. With an Archangel's Staff will add to your ap too.

Teemo- Twin Fang works with Teemo's poison
Twitch- Twin Fang works with Twitch's poison
Singed- Twin Fang works with Singed's poison

Leblanc- Once she hits lvl 6 she can silence you quickly with her q and ult in some cases kill you try not to be focused. If you can survive her burst she is easy to kill if you keep her poisoned and keep hitting her with Twin Fang
Kennen- Can be hard to get poison on him when he is using Lightning Rush Zhonya's Hourglass's active can help not get killed in a team fight. Also keep him at a range and pick him off with your poison and Twin Fang

Buy more survivability items if you keep getting killed by burst champions. If you can survive their initial burst you could easily kill them because Cass is more of a magic dps champion

To be continued

I really hope you enjoyed this guide. I didn't want to make it long and boring but i spent a lot of time making it. Please leave a comment for why you voted the way you did or anything I need to improve this guide.