Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Skill Reason: I like to max Noxious Blast first because of the amazing harass and minion (mage) farming ability with low mana costs and little risk to your person. I choose to max Miasma before Twin Fang because in conjunction with Rylai's you can easily gank or chase people.
Items: After you finish the six items (Sorcerer's Shoes, Haunting Guise, Rylai's, Lich Bane, Abyssal Scepter, and Zhonya's) I usually sell back the Haunting Guise for either Void Staff or Will of the Ancients. If truely needed, a high armor item such as Thornmail. The ONLY time to keep Haunting Guise is if the other team neglected their Magic Resist. If they did you have 48.55 Magic Penetration that will ensure you do massive damage to anyone without it. The switch to Void Staff is if they did not neglect their resist the bonus AP and Magic Penetration will be much more viable than 180 HP. If they have OK magic resist or constant damage that is nonlethal (non-spiking) the WotA is amazing for multi-hero fights.
Strategy: Spam your Noxious Blast early. It does good damage and once you hit 5 stacks of your passive it is easy to regulate mana. Gank at six by coming out of the bushes behind the enemy as they run from the poeple in lane. This assures that they will have to go through your allies again or get stunned by your ult. In team fights throw out as many noxious blasts and miasma's as possible around the tanks. Use twin fang to kill the softer enemies.