10/15/11
First ever build so please don't be too hard on me...

Cassiopeia is a lot of fun to play, but she's hard to use because of her skill shots and because her abilities are poison based. That means damage over time ( Teemo and Singed players know what I'm talking about) only harder to hit with. However once you get used to her she can dominate solo lanes and WRECK people.

PROS

High damage potential
Good harass
Skill spammer (check out her passive

Deadly Cadence
AOE damage (dat ult for example)

CONS

Hard to play
Mana dependent
Skill shots baby! BTW... Her Q is used like

Karthus Lay Waste so it automatically activates wherever your mouse cursor is as soon as you press Q
Poison doesnt do immediate damage so can be weak at times
SQUISHY TO THE MAX... play aggressive but also play safe or you'll have to b or worse... don't be the feeder everyone hates! (I've had those games... it sucks)

My runes:

Marks: magic pen x9

Seals: flat mana regen x9 (can be replaced by armor runes or AP (flat or per level))

Glyphs: flat AP x9 (can be replaced by CDR runes or MR)

Quints: flat AP x2 and magic pen x1 (because personally I like having at least +10 magic pen... up to you)

My masteries

I go 9/0/21 as seen above. Pretty straightforward for most casters although I like having 2 points on Utility Mastery because blue on Cass is the best thing ever and keeping it for 30% longer is even better
[/list]

]Here's my item list:

First, I go with [Doran's Ring. Doran's items are always solid starting items and the mana regen, AP and health are nice to have.

Next, rush Rod of Ages. I love this just because its strong and then gets better over time (whoa no way...) but its really good for the health and mana. You become less squishy and can spam more. I usually get Catalyst the Protector first on the way.

After getting your ROA (and your Sorcerer's Shoes in between) I go for Rylai's Crystal Sceptor because not only do you have a ton of health now (good for lasting longer in fights) but you also get the slowing effect, which makes you really good at chasing.

To finish, go get Rabadon's Deathcap (your damage shoots through the roof) and Will of the Ancients (spell vamp over time because of your poison!) You can get these earlier if you want.

Other good items on Cass are Void Staff (for the magic pen), Morello's Evil Tome (CDR is good for Cass but her cooldowns aren't that huge, especially if you get blue buff), Mejai's Soulstealer (you know...), Archangel's Staff (if you want to build up on mana), and Abyssal Mask (which is very good, I'd probably get this after everything else if I ever got that far...)

As you can see, I like my damage dealing items... but feel free to get Banshee's Veil or Moonflair Spellblade for defense.

I used to go Q,E,W,E,Q,R and so on maxing out Twin Fang first (see above for my more recent sequence). Then, I figured maxing out Noxious Blast was much better for harassing, just because after a while your opponent will probably stay far away from you... that and the damage becomes ridiculous...
Also, I get 1 point for Miasma at level 2 now because its really good for farming (like Morgana's soil thingy) and it slows enemies (setting up your Q or if you're chasing/spamming). Another good thing about your W is that you can use it on bushes and you can see enemies that are near the poison temporarily (a one second ward). Its nice.
Ultimately, E is the best because it sometimes has its cooldown reset (see ability) so you can dish out a ton of damage.
That just leaves your crazy snake woman ult Petrifying Gaze (you know... like Medusa right???). Only stuns if they are facing you or a huge slow if they are facing away. Either way its a huge AOE nuke and is very important in team fights. HOWEVER be aware that 1. It isn't a very long range and 2. if they dash at you (like stupid Kat's Shunpo) or flash, you might miss. You don't want to miss... especially if you've ever seen what Amumu's Curse of the Sad Mummy or Annie's Summon: Tibbers crazy bear can do to a fight.

SUGGESTED COMBO:
(After lvl 6) R, W, Q, E, E, Q, E, E, Q, etc. while tossing in ignite and some randomly thrown snakes (her autoattack) but most important thing about this is to get as much out of your skill shots as possible before spamming E. *Use of E depends on whether or not the cooldown gets refreshed

Pretty standard spells for casters:
Ignite for finishing off enemies (pairs well with poison) and Flash for placement and escape.

Other spells that are ok but I usually don't use:
Exhaust I used to use it but now I like ignite better. If you like chasing or if no one else has it for some reason, take it.

Teleport It's ok but if you like getting back to lane quicker or whatever go for it.

Ghost It's good for chasing but you'll be fine if you have Rylai's

Clarity and Cleanse are viable, but later in the game you won't need clarity and if you stay in the back and spam well from there, cleanse won't be necessary either.

EARLY
Get your Doran's Ring and Noxious Blast (Q), head over to mid/solo top (where Cass is most effective... but 2v1 lanes aren't fun so try to get mid). Your Q can be good for last hitting but its much better if you try to harass with it. Try to lead your shots because there is a short delay. Once you get Miasma (W) farming will be even easier, but it also helps you land your Q. At level 3, you should have Twin Fang (E) also. Your harass is even better (maybe sneak in a kill?) at this point. Watch your mana because despite your passive IT WILL DISAPPEAR. Once you hit 6, you can probably flash in and combo with Petrifying Gaze and whatever spam combo you got for the kill. Try to get blue early to help with laning.

MID/LATE
Cass is not a great ganker unless you hit with an ult. Shes more of a chaser/person who waits in the bush ready to stun that mofo whos trying to gank your teammates. In small or big teamfights, stay in the back and spam skills. You may need to get closer to ult (also make sure they're grouped up before ulting). By now you should have decent AP and a ton of health so don't be afraid to stick around in the fight (unless your team dies then gtfo). Make sure you throw a W onto the enemy to slow them after ulting (that and Q to maximize poison). While W slows, hitting with Q INCREASES YOUR MOVEMENT SPEED so you can chase or run away. During late game, things will be chaotic but you'll probably have a buttload of AP so as soon as a team fight starts, ult at the right moment and faceroll the **** out of everyone. You'd be surprised how much damage your poison will do over time given enough AP.

So that was my first guide! It was fun to make and hopefully this works out for anyone checking it out. The main goal for making this was to get my feet wet and try doing this for the first time so maybe I can try making more. Also, helpful comments (emphasis on helpful) would be nice because then I learn new things that might work better and maybe other people will pick that info up too. Let me know if anything is unclear or doesn't make sense and I'll try to update this ASAP. GLHF with Cass!