This works for me. I think it'll work for most people more often than any of the other guides on this site. I don't like it when champs feel hyper binary. I work hard to make guides that make champs more successful more often. I've got nothing against those people who play maximum damage and are successful with that, but I really don't think that on the whole League of Legends builds of that type are as successful for the majority of players. People who are good enough to build raw damage don't really need guides, nor do I typically see them building raw damage.

I hope you great success, and fun with Cassiopeia. I know I have fun with Cass.

When she first came I bought Cassio, and I played her and played her. I liked the DPS feeling, and back then the typical metagame had a ranged mid who you were almost guaranteed a level 6 kill on. I even built my first guide on Mobafire when my friends suggested that my Cass was good enough to share. After a while I felt that Cass was seriously underpowered compared to other AP champs. I waited for the buff. The buff came, and still I just didn't find her all that good comparatively.

I tried Rylai's, I tried Spell Vamp. I tried most everything to play the cool snake lady. But...

You could get a very similar effect in the midlane by playing Karthus and have several additional perks. Your ult is like a watered down Sona. Your damage is like Ryze without the survivability.

I wanted to play this champ though. I've been playing her once a week since, and I finally found a breakthrough that has allowed me to do my job and carry.

I tossed on my masteries from Volibear and picked up a Sheen first.

This build is not OP.

I think it's a viable way to carry normal games.

This champ is not OP.

I do not suggest this champ over any other AP.

I actually suggest that you get this champ after you have mastered Karthus' Q and Sona's R as a trainer. Even then, only get this champ if you really like her model, or are looking for a new challenge. It takes time and practice to get good with this champ.

Greater Quintessence of Healths are a must. If you have these and a Doran's Ring you will be the highest hp member of your team.

Greater Mark of Magic Penetration These are important as I don't typically recommend the Sorcerer's Boots.

Greater Seal of Armor Really most AD will shave these off, but it's enough to keep you from going into negatives at low levels. You can replace these with: Greater Seal of Replenishment.

If you go Greater Seal of Replinishment and you lane an AD, you have to hit those Qs early and often. You've traded that survivability for a couple of extra shots a minute.

Greater Glyph of Cooldown Reduction this helps when you get a gank early. Just drop everything at the opponent, just a hair faster. You could also go Greater Glyph of Magic Resist to help lane APs.

HP and Survivability. This champ is far too squishy.

You could honestly take Enlightenment and Evasion instead of Hardiness and Vigor .

Initiator is the bees knees though for Cass. We want to focus on staying in exceptional positioning while dropping AOE dps in team fights.

Here is the core of what is different about this build.

Early Game

Sheen: Very few champs will proc this more often than Cass. In the midgame or as a first item in dominion you will just be getting tons of free damage. You're basically adding 80 ish damage per second for free to the champ. Plus adding some early mana and ability damage. The price for this damage is unbelievable. You would need a legendary teir item to get 80 more damage per second out of her Q. The price to damage is very very good. Eventually we'll make a Lichbane, but we don't need to rush it because at that point our ability combos will make the additional hundreds of damage from it less of a percentage increase to overall damage than the sheen was.

Damage Examples at level 5 (assumed Doran's Ring) in a 3 second encounter:
Blasting Wand Cass is going to hit Q for 200. Auto for 80. E for 86 x 3 for 256. Auto for 80. Maybe miasma for 35. Totaling 650.

Sheen Cass is going to hit Q for 188. Auto for 160. E for 77 x 3 for 231. Auto for 160. Maybe miasma for 30. Totaling 691.

The longer a fight goes the stronger by comparison an early Sheen is. It also adds an additional punch right after being silenced which can sometimes be the difference maker.
Cassiopeia's success and failure depends on positioning and successful baiting for the ult. We need the HP for baiting part.

Boots

Mercury Treads or Boots depending on your enemies. We will miss out on some damage from the Sorcerer's Shoes but we will live to do more damage.

Midgame

It is beefing up time. It's no longer 1v1. You're being called on to bait team fights for your ult.

Rod of Ages I tend to get my first one between the 15 and 20 minute mark. If you don't have one before 30 you need to rethink your farm strategy or be more wary of enemy junglers.

I pick up a second if I can before 30 minutes. Why do that? 1200 health and 160 AP. Finishing your Lich Bane now will only net you 40 more ap and the tiniest drop of survivability. If I start to get behind and have a Blasting Rod and a Catalyst the protector at 30 minutes I just finish a Banshee's Veil and a Lich Bane and move on with my build a little less targetable.

Late Game

Lich Bane needs finishing.

Grab the Rabaddon's Deatchap for the late game umph.

Finish with a Will of the Ancients, Guardian Angel, or Force of Nature. GA is good for that final win push. FoN is good if teamfights are starting to last a long time (two very tanky teams typically). The WotA is good for giving certain team comps a little bit more late game.

Why no Rylai's?

Because she can only get the small proc. 15% slow is almost negligible to any Tenacity item. It seems wildly expensive to not have a single move that gets the full bump. Read any Morgana guide about the Rylai's. Same principle applies. Bumping up Miasma a few percentage points early just doesn't seem worth it at all.

Last hit with your auto-attacks. Tap your lane opponent with Q and then an auto attack. Keep tapping him. Try to keep your mana above half. If your opponent gets below one third. Drop miasma behind them and then Q on their model. Twin fang and auto attack. Hopefully you've got a kill.

If your opponent leaves the mid lane, miasma the caster minions and a Q them down while auto attacking the melee minions. Q the melee minions. Push the lane into the tower. Think about where your opponent has gone and head that way.

You never want to be first leave. Always get a wave or two of minions if your opponent leaves because you can do it quickly. Then if your presence can make a difference show up. Follow your lane opponent. You thrive when you can hit multiple opponents with a single Q.

Grab two wards every fountain trip and set them at your the jungle river intersections closest to you.

If you're having serious farm trouble pick up a philosopher's stone. Later it can be made into a Reverie which synergises well with your ult.

Grab an oracle's elixer.

There are two reasons for this. First with the midlane farms and a Rod of Ages, you can afford it and still be more difficult to kill. Every team needs an oracles midgame to clear wards. Second, you actually want to increase your target priority. You want Akali or Shaco to want to kill you.

You want their team to be bloodthirstily moving towards your position. That way you can hit priority targets with your ult. Try not to use your ult if you're only facing tanks. Try to use it when you've got a couple of squishies in the area and some of your guys with you. Akali is a great target for this. Even if she manages to turn around drop her shroud, miasma the back of the shroud and Q where you think she is. You'll either kill her or force her out of the fight.

Just focus on hitting those Q's on nontanks and racking in twin fangs on any squishes you catch while staying out of their Tanky DPS' zone of control.
If you're running away remember to lay miasma and q the opponent. This should get you out of most jams.

Remember your speed boost from Q. Q and move always.

Your ult is a great set up for other AoE teammates ults. Try to save it for when Kennen, Sona, or Amumu are near so you can get multiple free hits parties in a row. If one of these champs keeps leading with their ult, save yours and go second. You probably won't get a stun after a Mumu ult but you'll slow them down for the finish.