Introduction
Hello everyone! My League of Legends Alias is SmashedHippo and I'm here bringing you my first build/guide for Cassiopeia. Before I go into any detail, I'd like to note that this guide is subjective to my play style. It can however, be modified to satisfy one's preferences in game.
Key
This is what some of the terms used in this guide mean:
AoE = Area of Effect
DoT = Damage Over Time
Ult = Ultimate
Mpen = Magic Penetration
AP = Ability Power
Mp5 = Mana per 5 seconds
HP = Hit Points or Health
Lv = Level
CDR = Cooldown Reduction
Pros/Cons (Why play Cassiopeia?)
Pros
- Great AoE in teamfights (Ult, Noxious Blast)
- Awesome ability to focus a target (Twin Fang)
- AoE slow
- One of the best DoT Mages in the game
- Insane damage on your Ult + Stun/Slow
Cons
- Focused in team fights
- Not durable at all
- Not an easy champion to learn to play
Masteries
Nothing really that notable about the Masteries chosen. The most important things to know are that the utility tree gives you CDR, which is important for Cass, and the offense tree provides Mpen which is described why it's important in the "Runes" section. I also invest 3 points into
Meditation
because it gives your mana pool a little more "wiggle room" for harassment.
Items
The item sequence for Cassiopeia consists of an early 
Tear of the Goddess, to charge and build into an Arch Angels Staff later in the game. Next, build a 
Rabadon's Deathcap, followed by a 
Rod of Ages for the midgame. Whether you choose a Deathcap or Rod of Ages first is completely situational. For instance, if you feel that you are being focused in mid game fights, and need a little more survivability, go for a RoA. If you are dominating the enemy team and want to make a speedy conclusion to the game, a Deathcap would accomplish that. A lot of people also think that by getting a RoA, they are losing ability power they could've had from a Deathcap. I've done the math, and with a fully charged RoA and the passive from your Arch Angels Staff, it gives you 101.75 ability power! I choose not to rush a 
Rylai's Crystal Scepter because its passive only slows by 15% for all of her abilities, excluding twin fang. Her speed boost from noxious blast already gives you a good chasing ability as well, so I don't feel that a Crystal Scepter in the early game is necessary quite yet because it costs a lot for what you get out of it.
Skill Sequence
For my skill sequence, I max Twin Fang first because it's your main source of damage output. It is important not to neglect getting 1 point in Miasma before level 6 because it can deter melee champions from chasing you, and it is an easier way of poisoning the enemy for your 
Twin Fang if for whatever reason, you can't land a 
Noxious Blast. Note that Noxious Blast costs less mana than Twin Fang and it poisons your opponent. Unless they don't know how Cassiopeia plays, harassing with Noxious Blast will deter them from engaging you because they know they'll get 2 Twin Fangs in the face. It is also more mana efficient to keep your passive stacks with Noxious Blast because it costs the least mana and you'll have 50% the mana cost on your ultimate and your Twin Fangs - the abilities that cost the most mana.
Summary
I hope I have helped you improve your Cassiopeia play! Please leave your feedback in the comment section below (I do read it) and rate my guide :D! I will continue to update and improve upon myself, Thanks for reading!
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Build Guide
Discussion (1)
Greater Mark of Magic Penetration, 9 
Greater Seal of Ability Power, 9 
Greater Glyph of Scaling Ability Power, and 3 
Greater Quintessence of Ability Power. Mpen is especially important for Cass because her Noxious Blast is a damage over time spell, so the less Magic Penetration you have, the less damage you will do because the poison inflicts damage multiple times. You may also be puzzled as to why I have chosen flat ability power seals instead of mp5 seals. I do this because after playing Cassiopeia and weighing the pros and cons of each, buying a 
Meki Pendant & her passive usually eliminates any mana problems in the early game. Some may think that buying a 
Doran's Ring instead with mp5 runes yields a better early game in terms of power and hp, but I maintain that buying a meki pendant is better because 2 health potions will keep you in your lane longer and it builds into your Arch Angels Staff.
Clarity. You may want to consider 
Teleport if you're in a solo queue as your team mates may not hold mid for you while you buy items. I prefer not to take 
Ghost because your 
