First of all, this is a build and build explanation, this will not teach you how to play Cassiopeia as a champion, it will simply teach you how and why to build her in this way. Secondly, you should not follow this guide or any other guide to the letter, use your brain and forgo those Sorcerer's Shoes in favour of Mercury's Treads if you're up against a Veigar.

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Pros + Insane burst damage + Great ganker post 6 + Hard to gank due to her move buff and slow + Some of the best harassment out there

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Cons - Weaker late game than many AP carries if denied kills - A bad ultimate can cause your team to lose a team fight - Extremely skillshot heavy

• Greater Quintessence of Ability Power: Huge aid to your early game harassment, really important for Cassiopeia
  
• Greater Mark of Magic Penetration: Just about the only viable mark on most casters, takes you a little bit closer to true damage
  
• Greater Glyph of Scaling Ability Power: Nice late game boost to AP
  
• Greater Seal of Scaling Mana Regeneration: Allows for heavier harassment, can be replaced by greater seal of replenishment for more early game emphasis

Summoner's Wrath 5 AP when ignite is on cooldown, more AP!
Brute Force Easier last hitting
Butcher Even easier last hitting
Sorcery Cooldown reduction and to gain access to tier 3
Arcane Knowledge A great damage boost
Havoc A small damage boost and to gain access to tier 4
Blast A noticeable late game AP boost
Archmage A small Rabadon's Deathcap?
Executioner Possibly the best mastery in the game for anyone hoping to do damage, amazing on Cassiopeia


Summoner's Insight Flash more often
Expanded Mind To gain access to tier 2
Swiftness Not really useful unless you're maxing it, can be moved into Improved Recall .
Meditation More mana regen, more Noxious Blast casts
Runic Affinity Longer blue buff, amazing

Great for chasing, escaping, ganking, stealing, absolutely mandatory on Cassiopeia.


Fantastic damage over time with a healing reduction, useful all game due to the fact it inflicts true damage.

Can replace Ignite for split pushing, teleporting to wards for ganks, saving somebody from a gank or even getting back to lane quickly if you are being poked back to base.

• : More mana, more Noxious Blast casts, more harassment
  
• : Incredible lane sustain for what it costs, combined with Chalice of Harmony, you should be able to out-harass your opponent with ease
  
• : Brings your damage closer to true damage
  
• : AP, health and a great slow on twin fang, what's not to love? Oh, and it stacks with the slow from Petrifying Gaze!
  
• : Crazy AP boost
  
• : Allows you to q-w some minions to save going back to base to heal, works on nearby allies too! Great vs all comps especially poke comps
  
• : Adds AP, magic resist and cooldown reduction to your Chalice of Harmony, I get this last as Cassiopeia isn't heavily reliant on cooldown reduction

• : Incredibly cost efficient and provides early game late domination
  
• : Brings your damage closer to true damage
  
• : Needs no explanation, late game this will be taking off close to 50% of someones health not counting magic resistance not to mention the 90 AP it gives you
  
• : Crazy AP boost
  
• : AP, health and a great slow on twin fang, what's not to love? Oh, and it stacks with your ultimate!
  
• : Allows you to q-w some minions to save going back to base to heal, works on nearby allies too! Great vs all comps especially poke comps

Credits to jhoijhoi for the template, which you can find here.