While her sister Katarina has always been the most celebrated member of the household, the Du Couteau family has a long history of service to Noxus. It has often been said that no soldier has ever been as fortunate as General Du Couteau to have been graced with daughters. His youngest, Cassiopeia - despite lacking her sister's killer instincts - was equally renowned in court for her stately character and elegance. Cunning as she was beautiful, the temptress could never be found far from the arm of any foreign dignitary, her wiles prying secrets from the lips of even the most wary attaché. With the Noxian barbarian pacification campaign having ground to a standstill, Cassiopeia had set her sights on a diplomat from a tribe of the Freljord region. Thinking him an easy mark, the scheming seductress set about beguiling him. He refused to confide in her, however, until she swore an oath of secrecy upon his sword - a strange weapon with a serpentine curve to the blade.

Once her tryst was over, Cassiopeia provisioned her father with information regarding the barbarian resistance. As she divulged this intelligence, a wave of revulsion washed over her. She screamed in agony as her silky skin hardened to scales, her lustrous hair thickened to leather, and her manicured fingernails sharpened to claws. Dazed, she fell upon a group of horrified servants, rending them limb from limb in a heartbeat. When it was over the blood-soaked figure was no longer the ravishing jewel of the Noxian court, but a horror trapped somewhere between woman and serpent. Unable to serve in her traditional capacity, Cassiopeia departed for the League, continuing her service to Noxus on the Fields of Justice.

"Though she may have appeared the innocent flower, she was the serpent under it." - Katarina, the Sinister Blade

PROS
Sexy Voices
Great scout
Many escape mechanism
Most powerful mid game champion early on(Well played)(Atleast i think so becouse i owned all enemies easy with her)

CONS
Kinda fail damage late game(Thats why i take Mejais soulstealer 'n Rabadon Deathcap early on so i get fed like hell + Ability Power per level runes)

Deadly Cadence
After casting a spell, any subsequent spellcasts will cost 10% less for 5 seconds. This ability stacks up to 5 times.
-Great passive.Keeps your mana stacked up.

Noxious Blast
Cassiopeia blasts an area with a delayed high damaging poison, dealing 75 / 115 / 155 / 195 / 235 (+90% of ability power) 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.
-I take it at level 1 but keep it there.
Even at rank 1 its still great harrasing tool + Becouse of low cooldown you can get your poision stacked on target allowing you to spam twin fang.

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 (+15% of ability power) magic damage each second and slowing them for 15% / 20% / 25% / 30% / 35%. Continual exposure renews this poison.
-This is great ability which keeps poisioning your target.I take it at level 2 and max it by lvl 13

Twin Fang
Cassiopeia deals 60 / 95 / 130 / 165 / 200 (+55% of ability power) magic damage to her target. If the target is poisoned then Twin Fang's cooldown is reduced to 0.5 seconds.
-Main source of damage.I start maxing it at lvl 3 and since then max it first.
Petrifying Gaze
Cassiopeia deals 200 / 325 / 450 (+60% of ability power) 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% for the duration.
-Great combo set-up

Open up withSpam [on enemy while they are in.
If they try to leavearea(They sure will) Stun them withAnd keep yourspamming on.
Once they finally leaveUse yourthem-spam combo till they die.Also use yourif you need.This is really hard to remember that makes Cassiopeia so difficult to play.But if you remember this combo you pretty much can play her good.

Health 1730
Mana:1150
Speed:310
Armor:83.5
Magic Resist:30
Critical Chance:0%
Health Per 5 Sec:13.85
Mana Per 5 Sec:14.5
Range:550

I like her Health and mana,and regenaration and armor and everything.Only that her magic resist is kinda low while noticing she has great range.

Also hard thing about Cassiopeia are her item ways.
She sometimes need to get more agresive more of a disabler or active mage.
My personaly favorite active mage:




or

depending on enemy team

While Cassiopeia with Heavy CC:





While if you wanna play Cassiopeia anti-squishy grab

(Magic Penatration Marks)becouse all mages need penatration.


(Mana regen seals)cominized with my passive,dorans ring and passivly mana regen it will not be easy to get OOM.

(Ability Power Per Level Glyphs and quintessences)Becouse she doesnt need lots of AP early on becouse then she has great damage output with her abilites but later on she has kinda 30prezent damage failior so (per level ap runes) are better then (flat ap runes) on Cassiopeia

I focus on regenarations,cooldown reduction,movement speed and i grab imrpoved flash!
In ofensse i take ability power and magic penatration.

Those spells work on Cassiopeia.Others basicly dont work.
Ghost-Utility
Smite-If you wanna jungle on Cassiopeia
Flash-Great for setting up your ultimate which is low range while Flash gets you in it.
Ignite-Maximazing your overtime damage.
Cleanse-Againist heavy CC Team.

Takeandat begining after those items start one of those 3 builds:Active mage caster,Antisquishy mage
or heavy CC Cassiopeia.
Skill Order change to:and then follow my ordinary Skill Sequence.

Thank you for reading this build.Hope it will come useful to you.
Vote and comment after testing please.