Pros

Great for lane control.
Many enemy champs ignore the poison at early levels and stand in it because everyone else loves Twin Fang so much.
A lot of early kills due to enemy ignorance.
High end game health with hp/lvl runes.

Cons

Slightly more squishy than normal early game.
Must play cat and mouse in your lane a lot. You aren't the cat. =P
Ranged champions are more adept at dodging your poisons

Please make sure to think about how you play before testing this build. This is definitely rough to do if you are used to going straight for kills. This is more like playing Singed than anything.

Okay, so I know I am going to get a ton of negative feedback right off the bat on this one. Everyone seems to believe that Cassiopeia has to use Twin fang as soon as possible. I have actually found out that if you build up her Q (Noxious Blast) and her W (Miasma) you can keep almost any team at their tower and own them that way. Now for those who are used to a full on attack play style I wouldn't suggest this build.

Like a few other Cass builds I've seen so far, this build is a weak hp build in the beginning. Just be warned. You will be running away a lot.

Okay, so for the runes I made it really easy. They are as follows:

3 Quintessence's of Insight (+1.89 Spell Penetration)
9 Greater Mark's of Insight (+0.95 Spell Penetration)
9 Greater Glyph's of Intellect (+11.25 Mana)
9 Greater Seal's of Vitality (+1.08 Health per level [+19.44 at champion level 18])


For Cassiopeia's poisons to do any REAL harassment damage you absolutely have to have spell penetration. While it might not be too bad in the early game you need the spell penetration for champions that are tanks. Lots of health means if they have any magic resistance they are going to be hell against you without any kind of spell penetration.

You need extra mana early on too. With this build you are going to constantly be casting and there is a necessity for lots of mana and stuff to bring it back. Clarity is going to help, but you still need the extra mana to stay in the lane longer.

The build of masteries on Cassiopeia is very similar to any other caster build. 9-0-21. Make sure to go and get your 15% spell penetration from the offense tree and then go all the way down the utility tree for more cooldown reduction and a better recharge time for your spells. The way I built the utility tree is my own personal preference, but I have seen it built in a couple other ways. Feel free to play around with that a little bit.

I would strongly suggest that as you go down the utility tree you make sure to pick up the increased experience gain. It would benefit you to have a bit of a level bump early on so you can get health faster.

This is where I'm sure I'm going to get a lot of flack for my build. Yes I know that I don't pick up Twin Fang until level 13. I've found that with well placed Maisma's and Noxious Blasts you can hold off up to 4 people in a single lane on your own for a bit. This takes a lot of movement and running away at times, but the damage they take over time from poisons is well worth it.

*******EDIT*******
Just for your information, the AP scaling has gone up for twin fang from .45 to .55 as of Tuesday, January 4, 2011. If you feel this is enough of a change to go with at least one point of twin fang early on, then put it in at level 5. It's probably the best way to go if you are interested in this build at all. The extra spike damage will keep you from running away as much in the early game.

Clarity is a necessity early on with this build. Sure, you have extra mana from runes, but to harass the enemy champions in your lanes properly you are going to be using Noxious Blast and Miasma a lot. Therefore, the extra boost of mana will keep you in the lane longer so you can do more damage to the enemy tower and champions. Ghost is a great spell for this build mainly because you are going to force the other team to their tower which makes it more likely that their teammates are going to try to gank you for the annoying harassment. If you prefer Flash then go ahead and use that instead. It's really just a preference game when it comes to that anyway.

Also, remember that while running away your Noxious Blast gives you a movement speed increase if you hit a champion with it. That mixed in with Miasma, Petrifying Gaze when you hit 6 and ghost/flash makes for great survivability during gank times.

At level one with this build you are going to be very low in the health department. Basic Movement speed 1 boots and 2 health pots and a mana pot is suggested.

From there you want to start the build on your Rylai's Crystal Scepter. I would suggest that you build up the AP items before the Giant's Belt. Your runes for hp will start kicking in until you can get it. The more damage on your poisons; the more they have to go back to the base and heal.

After the Scepter is built you will probably notice that you need to start moving a little bit faster. It's time for your spell penetration boots.

From there you can pretty much build the rest of the items in any order. To me it's usually between Void Staff and Zhonya's Ring. If you have someone like Galio that builds Magic resist quickly I would go with the Void staff. Otherwise go for as much AP as you can get and build your Rabadon's Deathcap. After those two items are built it's all gravy. Get the rest in any order you want.

Everyone, Thanks for checking this build out. It's my first build here on the site and I would appreciate any feedback I could get. Still having a bit of trouble with the coloring of the headlines, but other than that it seems pretty straight forward.

*****EDIT*****
(If you are getting outlaned because of a drastic difference in movement speed you can also switch this out for Movement 3 boots, but in that instance make sure you get the Void Staff ASAP.)

For those of you who actually test this out I wish you all good luck. It works REALLY well for me, but to each his own. =)