10/16/11 - Guide has been made! My first guide for anything ever, so be nice :P

Cassiopeia is, in my opinion, one of the best AP carries currently available in LoL. Her damage output is incredible, and she has one of the best ultimates in the game if you can use it correctly. She can go mid and hang with anyone, whittling them away with her constant damage. If you know how to do it, she is one of the best 1v1 champs in the game, and she's really good in teamfights, too.

Cass is a solo lane champ. She can mid or top, but I prefer mid for whatever reason. She's capable of jungling, but I have never personally tried it, and I can't imagine it's too strong. I would not put her in a duo lane, either. She's not a support, and she's too good at sustained harass to just let the other team's support heal it all away. If your team isn't running a support, I still wouldn't run her bot. She needs all the farm she can get.

Before you go on to the meat of the guide, I'm going to say that Cass is a difficult champ to play. There's no denying that. She's one of the best when she's used to her full ability, but it takes a lot of practice to use her at that level. That doesn't mean she's not fun, though! She's my favorite champ for that very reason. Watching yourself do all that damage is great.

MARKS: Magic penetration. In my mind, the only viable reds on a caster. They help you early, and they carry over to late game. With your sorc boots, you'll be cutting right through that MR.

SEALS: Mana per 5/level. Again, I use these seals on all casters that use mana, the spammability you get in the later levels is too nice to give up. You could also consider health/level or even AP/level if you wanted, though.

GLYPHS: AP per level. Cass's skills all scale with AP, and getting more as you go on means you'll be that much more of a threat late game. You could consider MR/level, but AP scales better and you can outharass almost anyone anyways. CDR is a bad idea on Cass, though, because her skills are spammable enough without losing the damage for more CDR.

QUINTS: Flat AP. No better quints for a caster. You can start that harass from level 1, and the 15 AP continues to make a difference beyond that. If you're really having problems surviving, though, you could use health quints until you get better at harassing.

For masteries, I use the standard 9/0/21 caster set up. Get through the 15% mag pen in the offense, then go through the utility tree, being sure to pick up flash cooldown reduction. With this set up, you'll start with 9% cooldown (good enough for the whole game, since you should be getting blue buff anyways after a while) and decent move speed.

I get 3 in perseverance over 3 in good hands because you shouldn't be dying as Cass, you should be standing in the back spamming your spells. It's really up to user preference, though. It doesn't matter too much if you'd rather get good hands. Same with greed over meditation. User preference. My reason for getting it is, though, that with the build I use, the one mana regen you're missing out on is unnoticeable, whereas the 270 gold you make in a 45 minute game with greed is essentially 3 free wards.

Recommended

Flash - Flash is a must on every champion. It forgives you for getting ganked, you can flash to get that last hit in, or you can flash to position yourself for your ult. Simply a must, especially since Cassiopeia has no other means of escape or gap closing.

Ignite - Helps to finish someone off for a kill, or to cut someone's healing in half. It helps in 1v1. 400 true damage isn't something a carry can just shrug off late game, either.

Possible

Ghost - I would never swap flash for ghost, but I could see swapping it in for ignite if you're having trouble with your mobility or positioning yourself.

Exhaust - Another possibility. It enhances your 1v1 capabilities (which you really don't need help with) and slows (which again, you don't really need help with). But, if your carry is going ghost flash, it's a possibility.

Teleport - Teleport is more useful when you're going top. Cassiopeia can top, but I prefer to take her mid, so I don't generally consider it. But if you're going top, teleport helps you get around quickly since you aren't in the middle of the map.

Clarity - Some people use this, but when you get more used to playing Cass, you'll be able manage your mana better. That, combined with the tear of the goddess and the mana regen runes make clarity pretty much useless past level 8 or so.

I don't really foresee any situations that you would use any of the other summoner spells.

Before I get to the skills, I just want to make a note. I feel it's almost mandatory to smartcast Cass's spells. Cass's q is smartcast all the time, but I think the others should be too. This adds more to the getting used to her (since you have to memorize her ranges as well as regular positioning), but it works so much better once you get used to it.

SKILLS

Deadly Cadence - Your passive. After casting a spell, your mana costs go down 10% for 5 seconds, stacking up to 5 times. This is pretty helpful given your low cooldowns. If you're trying to fight someone, your q's and e's will have you getting the most of this passive almost all the time. Now, another thing I've noticed. This lets you farm your tear extremely efficiently. Once you get up into the teens of levels (or blue buff), you can start at base and walk to lane, spamming noxious blast on the ground. With 50% mana cost reduction, you'll be regenerating almost as much mana as you're using to use the spell. So that's an extra ten casts on your tear right there, basically for free.

Noxious Blast - At level 5, this is hitting for 235 + .9*AP on less than a 3 second cooldown. One of the best harasses in the game, and it's great for farming, too. The skill shot isn't that hard to hit, but there is a slight delay so it takes some time to get used to. Hit a couple, though, and your opponent will be hurting. Max this first.

Miasma - Useful to farm, and synergizes well with the rest of her kit. The renewing poison is great for 1v1's, and it slows if they're running. It also grants vision, so you can use it on bushes and the such instead of facechecking. I max it last, but make sure to get a point by level 4.

Twin Fang - This skill is fantastic. If the opponent is poisoned, the cooldown is reduced to half a second. A nuke on a .5 second cooldown. So much damage potential. This is not a skill shot like the rest of her kit, but it does require poison to be effective. So you do have to hit a skill shot to make this useful. I max this second.

Petrifying Gaze - One of my favorite ultimates in the game. There are lots of ways to use it, but to use it effectively, you're going to have to practice. You can use it in 1v1 for four free fangs. You can use it in a team fight to take the carry out of commission. You can use it when running away to get away. You can use it when chasing to slow them enough for the kill. It has nice damage, as well. The range is a bit tricky, though, and like I said, it's hard to use. Everyone's going to fire off a couple bad ones, don't let yourself get down. Just position yourself better next time, and you can pretty much turn a teamfight by yourself.

SEQUENCE

Petrifying Gaze > Noxious Blast > Twin Fang > Miasma

You're going to be farming and harassing with your noxious blast, so it makes sense to level it first. Some people max twin fang first, but you'll run out of mana if you use it too much early. By the time you max it, you should have a couple doran's rings, your tear, and maybe blue buff. Spam away all you want. Miasma last, because you just don't use it that often. I use it more often to facecheck and farm than I do to harass. If your opponent is dumb enough to stand still in it for the whole time, you can just as easily land two or three noxious blasts for more damage. Obviously get your ultimate whenever you can.

You can start one of two things - Boots + 3 Health Potion or Doran's Ring. It depends on who you're up against. If you're up against someone who's going to harass you like Kassadin or Brand (etc), then start with boots and pots. The boots help to dodge skill shots, and the potions heal up damage that you take. On the other hand, if you're up against a someone who doesn't do much damage, or is more of a farmer, take the doran's ring. It'll help with your harass and early mana problems.

First time back, everything is dependent on how much gold you have. If you've farmed well, if you died early, if you got first blood, all these things factor in. I try to get out of here with Sorcerer's Shoes + Tear of the Goddess + Doran's Ring, regardless of what I started with. If you don't have that much gold, improvise. I might get out of there with two doran's rings + boots if that's all I can afford (note if you can afford the tear you don't really need another doran's ring). Just make do with what you have, and don't forget pots and a ward or two. In solo queue, you can't count on a teammate to call efficient mias, so wards and map awareness save lives.

Now I try and get a Rylai's Crystal Scepter and a Will of the Ancients, picking up pieces as I go. By the time you get to this point, your lane opponent is generally starting to get some damage, so the giant's belt helps you take a little more (and have more time to dish some out in return). Life steal is great on her too, since her damage is so sustained.

After this (or possibly before if you're fed), I get my Rabadon's Deathcap. I find that I don't need it that early, because Cass does plenty of damage without it. She harasses and farms extremely well, even without it, so I tend to go for the utility and sustain before the damage. Like I said though, that's just me. If you're fed and want to get this earlier, go for it. Just don't come complaining to me when you're squishy as hell.

By the time you get your deathcap, you're killing people left and right if you're doing it right. Finishing your Archangel's Staff is just a slap to the face. Your tear should be nearly full by this point, meaning the passive + base AP + 30% from deathcap will increase your damage output by quite a bit.

Last, you can go for a Void Staff. By this point, the tanks have 200 MR, the carries have their banshee veil, the tanky dps has finished their FoN, etc. Magic penetration is never a bad thing, and it gives some AP too.

Note: You will sell your Doran's Ring's at some point if you get squeeze a full build out. This doesn't happen that often, though.

Builds in League of Legends are entirely circumstantial, of course. You can't expect to build this every game and be successful. If you're getting burst down by the Akali, build Abyssal Mask or Banshee's Veil. I'm not a fan of Zhonya's Hourglass on Cass (because that time you spent as a golden statue is time spent not sending fangs and poison at them), but it is a possibility if you need armor or are getting focused.

Thanks for taking the time to read my first guide! Cassiopeia is one of my favorite champs in all of LoL, and she has two of my favorite skins. She's lots of fun to play, and very VERY powerful if played right. I hope with this guide, I've pin pointed some of the main points you need to focus on to get to that level. If you have any questions or want help (god knows why you would want help from me, but still :P), feel free to add me in game, same name as here. I aim to keep this up to date.

Comments are more than welcome, and criticism is certainly not frowned upon. I look to be making more guides in the future, but I can't do a very good job of that if you guys don't tell me what to improve on! I hope you enjoyed reading!