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.
So this is my first build, thought I'd share a build that usually works really well for me.
For Spells I mainly use ignite and ghost. The reasoning is with the Burning embers mastery you gain 10 extra AP while Ignite is on cool down which helps out with your expunge and ghost is a great way to get in and out of combat especially if used with ambush.
Malady - nice lifesteal and attack speed boosts
Berserker's Greaves - further increases attack speed
Zeal - Bonuses to crit chance, move speed and attack speed I find very useful.
Phantom Dancer - Further increase Zeal and gives a dodge increase
Bloodthirster - the added attack power and lifesteal will keep you alive
Infinity Edge - Damage and Crit chance upgrades really help
for the final item I either get another bloodthirster or a Black Cleaver. Either one works great and can help decimate the enemy.
Play-style: To start off just last hit minions and play a bit passively. Once you get expunge then you can start playing more aggressive and really hurt your opposition. Keep this going till you hit level six and get spray-and-pray, then the ganking begins. Always be on the lookout for possible victims and once you find them, backdoor them and they wont have a chance. The same thing for team fights get behind the other champions and unleash spray-and-pray so that you can hit multiple targets, that along with expunge and Debilitating poison and I guarantee you'll get multiple double, triple and even quad kills during the game.