Hey guys, this is my very first build!
Dont flame me please....

Pros:
+heavy dps
+decent tanky
+pokes ain't that effective against Cassio
+with blue, bully in lane
+makes enemies into stone..nuff said

Cons:
-killed if stunned
-hard to play good
-vulnerable vs CC
-often banned in ranked

I use those runes on allmost every mage carry in the game! (except from Vladimir, Kennen, Rumble Akali etc.)
Theese runes gives you the opportunity to stay in lane forever!
Use quint of swiftness if you are meeting champions with skillshots such as Brand, Lux, Morgana etc. so it will be easier to dodge.

With maxing "q" befor level 9 become very usefull to give moederat pokes. Try to land "q" when your enemy tries to get a creep/minion.If you hit your "q", run and spamming "e".

In lategame you will carry your team, easy. You'll become really tanky and dealing heavily damage!
I prefere to get spirit visage after Will Of The Ancients. Especially if you are facing champs with snare, such as Morgana, Ryze, Lux and Swain.
Feel free to sell the Doran's Rings to make space to other items.

If you got a champ that can poison its really good to go cassio mid, since her "e" then have only 0,5 sec cooldown.
Its very important to land your "q" or "w" before using "e"!
Your can be usefull of useless. Make sure your enemies are facing you when you use it. Bait them into bush for excample.
Also make sure you get atleast the 2nd blue, then you can spam abilities as hell...

Thank you for checking out this build! :)
Summoner name: aalreit
GL HF with playing cassio!