Hi, this is my first build so don't expect anything very fancy, but I'll do my best.

Pros

Stealth
Can't be ksd if you use expunge
Can ks very good if you use expunge lol
If you position good with your lifesteal and your ulti it will be hard to kill you
Enemy can't run away from you cause your expunge has a very long range

Cons

Squishy early game
When you start attacking from your stealth you will be mostly targeted
Hard to master

Ok, lets do this

Start with Rejuvenation Bead and 3 health pots and go for mid. Your bead will heal you from some little harass and if you lose allot of HP use a potion and you can stay in mid a long time without recalling. When we get back we'll buy an Emblem of Valour for more HP regen and lifesteal (if you used your health pots buy more). This is when I mostly hit lvl 6 and try to kill, but that's a different chapter. We'll get Boots of Speed and Zeke's Harbinger if we can. After a Vampiric Scepter and Mobility Boots its up to you to get what you need. I mostly get Phage for health and dmg but you can get The Bloodthrister if you want to rush it.... I can't say anything more cause its up to you to buy what when. Its jut important to have Mobility Boots (if you really need it, get Mercury's Treads if the enemy has alot of CC), stark's and a bloodthrister early.

Well, Zeke's Harbinger is like a must-have for me. Its lifestea, health regen and attack speed, what twitch needs the most. [Boots of Mobility]] to go fast to gank top and bot or get back to mid before they eaven notice. The Bloodthrister is eaven more lifesteal and attack dmg for a scary twitch. So your scary eaven now but we must get more scary. With Phage you will get more health and dmg, with Frozen Mallet you will slow the enemy and it will be eaven more easy to chace them.

The offence tree: Its simply what we need the most, attack speed, armor pen., cdr and ad.
The defense tree: Your heal cd reduced by 30 secs? why not? Armor and dodge chance, same. Remember you die easy, too easy without some defense.

Greater Mark of Desolation: We all need armor pen.
Greater Seal of Defense and Greater Seal of Armor: armor early and late.
Greater Glyph of Magic Resist and Greater Glyph of Health: health and magic resist. Not all in armor ;)
Greater Quintessence of Movement Speed: Move like a ninja!!!

Wait for level 6 and emblem, but if you think you can get a kill before 6, do it. Here is how to do it: with your expunge you can harass very well with just one hit. When he gets low, stealth, get near him, ulti and attack!!! Ignite him, if you think your gonna die use heal when you hit him 6 times use expunge and that should be it. If you don't kill him ask your team for a gank.
You must be able to last hit allot, practice in custom games to last hit. Here's a tip: if your enemy runs away when you stealth try this: go in the brush to top/bot like your gonna gank and stealth, he will think your going for a gank and you can attack him easily. If the enemy has an oracle, gank!!

What I use:

Heal and Ignite cause your squishy and and your poison plus ignite... :P

Viable:

Exhaust
Flash

Stealth and try to hit them all with your ulti, nothing more to say.

Deadly Venom Passive: Ads a poison that does DoT for 6 secs and stacks 6 times when you hit an enemy its 1 stack and so on...

Ambush Q: You become stealthed and when your stealth ends by combat or time-out you gain bonus attack speed.

Venom Cask W: You slow enemy champions in a big AoE and the more stacks of your poison they have, the bigger the slow.

Expunge E: Has the big AoE like your W but it only hits the poisoned enemy unit and does dmg, you should wait in a close combat for you to add the max of 6 stacks of poison for the maximum dmg.

Spray and Pray R: Is like this: your attacks get a bigger range and when you do a normal attack it will hit all enemies in a line from you to the max range of that spell. So when you build lifesteal you want to hit many enemies for more healing from it.

Sorry for no fancy pics but this is my first build and I hope I helped you.

Remember: FARM FARM AND FARM

EDIT: 9th August, 2011: Removed normal pictures and made it easier to read.
EDIT: 30th August, 2011: Made the "Skill" chapter better.