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Not Updated For Current Season
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Somehow, you find yourself playing Cassiopeia. Congrats. She can be super fun to play, but she is... quite frail. I have played Cass quite a bit recently, and she is a very tough champion to play and master. I feel I have adequate knowledge on the champion to give you this short guide and to hopefully help you out in your pursuit of awesomeness.
Note: Most people, when they see you pick Cass, whine about your champion choice. This is inevitable. Cass is not the strongest mage out there, and it is very hard to carry with her. You will probably suck at first, too, so be prepared for that. If this has not scared you off, here is the guide:
Typical 9/0/21 masteries. I skip greed because Cassiopeia is a very good minion farmer. I prefer utility mastery to expanded mind because I don't really have mana problems at all, and though you will likely not be getting buffs for your team (neither buff is that awesome for Cass) it is handy when you kill someone who has buffs. Obviously the mana from blue buff is helpful, but seriously... as long as you keep five stacks of your passive (explained later), you will not have mana issues.
Magic pen marks, mana regen/lvl seals, flat cdr glyphs, flat health quints.
Magic pen marks are a must for casters.
I choose mana regen/lvl seals to make sure I have no mana problems. Could use flat mana regen seals, but mana regen/lvl pay off after about level five.
Flat CDR glyphs help with early harassment and ganking.
Flat health quintessences help with your very lack luster health pool at early levels.
Most Cass players max Twin Fang first. I, however, do not even touch it until the end. I feel Cassiopeia's poison abilities are the strongest abilities while laning and in early teamfights. Always max Q, Noxious Blast, first. It does very heavy damage and increases your speed when you hit champions. This is how you will be ganking. One good Q and you are outrunning the enemy, and you are able to slow them with your W.
I guess I should talk about what her skills actually do!
Q: Noxious Blast
Cassiopeia's Q ability, Noxious Blast, places a bomb on the ground that detonates after a short delay (a little less than a second). If it hits a champion, it increases Cass's movement speed. Awesome for harass. If you have ever played Karthus, forget everything you know about his Q! Cass is very different! While Karthus can spam his Q, Cass can not. You have a cooldown between casts. Your Q is going to be your main damage spell until late game. It is very effective because it has a long range, does very nice damage, and makes you move faster! The only problem you will have with Q is aiming. You should always test champions to see what they will do when you place your bomb on them. Some move forward, some move side to side, MOST move backwards. Test your opponents and see which way they move, and make sure you make a mental note about it.
Miasma is Cass's W move. When you first press it, you will notice that the outline of where the spell is going to hit looks weird. This is because Miasma grows as time passes. When you place it, it is the darker center region, and while it's duration reaches the max, it grows to the size of the bigger circle. Miasma only puts a two second poison on enemies, but it refreshes itself when they stand in the poison. It also puts a very nice slow on them. Miasma is great for teamfights and harassing at turrets. You can run in range of the turret, throw miasma down next to/behind the enemy champion, and make them run either towards you, which could get you a kill, or away, which means you can start pounding their turret.
E: Twin Fang
Twin Fang (E) is how most Cass players deal their damage. It is also why most Cass players suck. The range on E is so short, Cassiopeia is always in danger when she is casting it. Thus, I feel it is not a good harassment tool, or good for team fights. It is also a mana hog, so at early levels you will be running out of mana even with five stacks of Deadly Cadence. Late game, E shines as a very strong ability with almost no cooldown. I still rarely use it in team fights, but if you are picking people off one by one, killing dragon/baron, or if you have stunned someone with your R, E is a very effective ability.
R: Petrifying Gaze
Petrifying Gaze is Cass's R ability. After a quick cast animation, she stuns anyone looking at her and slows anybody turned away from her. This also does decent damage. Remember, when you cast R, THERE IS A DELAY! If you are stunned or they move out of range, it will not work. That being said, R is a very effective CC for Cass. 1v1 and 1v2, it is pretty much a guaranteed stun. In team fights, you can stun/slow many of the enemy players and use flash or ghost if things aren't looking good for you. More to come about her ult later.
Passive: Deadly Cadence
Deadly Cadence, Cass's passive, reduces the cost of abilities by 10% for 5 seconds after an ability is cast, stacking up to 5 times. This gives Cass staying power in the lane. More about this later.
Laning with Cassiopeia is very challenging. Always start with Meki Pendant. The MP5 is amazing, and will lead you to have no mana problems if you play correctly. Cassiopeia's laning phase relies on keeping her passive ability stacked at five stacks at all times.
Working with the passive
Her passive ability, Deadly Cadence, makes it so that every time you cast a spell within five seconds of the last, it reduces the cost of subsequent spells by 10% (max 5 stacks, 50%). This is the hard part! You will see the buff on your buff bar every time you cast a spell for the first time. You will see it's duration run out. This looks like the buff is being shaded in a clockwise fashion. To maximize your mana, you don't want to use Q every time it is up (unless you are harassing the enemy champion, do not use it every time it is up on minions or out in space), you want to use it every time this buff is about to expire. This means that if nothing is in range, you use your Q on the ground somewhere just to keep your five stacks up.
Using her Q
If a champion is in range of your Q, hit them. Even if your passive has 3 seconds left on it, harassment is how you are going to get ganks. If no champions are in range of her Q, use it on minions to try to get last hits, coupled with your auto attack (assuming the enemy champion is not in range). Cassiopeia is VERY frail, so taking unnecessary damage often times leads to you dying.
Using her W
Cassiopeia's W ability gives her amazing zone control, and lets your harass while you are pressed against the enemy tower (which is NOT an ideal situation, but is inevitable sometimes). Later in the game, her W is an amazing minion destroyer. Early game, however, it is a mana hog and should be used sparingly. I mostly use W when the enemy champion gets a bit too aggressive. Again, Cass is a weenie when she gets hit. Using her W to slow and her Q to speed you up, you can outrun almost anyone early game. Once they get hit with your slow, they usually back off.
Using her E
I don't get E until very late game. It is too risky to use it in lane, as the range is very short. It is also a mana hog. Use it only when safe. Do not go into range just to use E, unless you are sure you are going to get the kill and slither away alive.