Hey everyone, this is my first real guide. I should start off by saying that this isn't really an aoe build until your 4th or 5th item when you get Tiamat. Overall this is a very standard AD Twitch Build with exception of Tiamat. If your looking for a strategy guide for Twitch, this isn't the guide for you. This is more of an item that should be known. Most of this goes for Dominion where I've tested it out. It could also work for Normal because of the phenomenon in the Summary section.

My runes are just what I like. You can choose w/e you want. I find arpen marks, as seals, and cdr glyphs with arpen quints the way to go. Normally, having anything tanky like armor seals or mres glyphs don't really help that much. Your squishy and you will normally die either way. With the as seals you get 1 more lifesteal hit off which will allow you to stay alive just as long as the armor/mres. The cdr is a little more debatable, but I find that the earlier I can re-stealth or expunge the better. Yes attack speed is possible, but after the seals and all the attack speed you will have anyways you won't need more.

You may wonder why 22/0/8. The reason is you have to have the xp in utility. As someone who depends on staying ahead, the xp you get every second with this bonus means you will hit 6 before others do. When you hit 6 you are a big help in team fights which is what Dominion is. The Magic Pen may seem unnecessary, but your e ability is

The items are all very self-explanatory. The only one I need to explain is Tiamat
This item may see weak and unneccessary, but it is god-like in team-fights. Let me explain with some stats. In a teamfight on Dominion there are usually around 2-3 champs, and because you are very squishy you normally can only get 3-4 shots off before you gets slaughtered by stuns and what-not. For an average we will say 3 champs and 3 shots before dead.
Normally:
371 damage with 55% crit and 260% crit damage
This means
(371*2.6+371)/(100/55)=734.6 per hit on average
734.6*3= 2,203 in 3 shots.
This is pretty impressive. For squishies they are dead, but if your facing some tougher champs this won't cut it. 100hp for them is more than enough to mow you down if they have a stun.
Here is what Tiamat does
734.6*0.5*3+734.6= 1,837 per hit per champ
When you multiply by 3 for the 3 champs you get
1,837*3= 5,511
I don't know if that has sunk in, but this is roughly 2.5 times as much damage as w/o Tiamat. Now of course I was just doing the stats w/o a replacement of tiamat, but no item can increase your damage by that much for 2,050g. Now of course if there are 5 champs all in one spot, then the damage gets even larger. Just for kicks it would be:
734.6*0.5*5+735.6*3= 7,713 If we add in total damage then this is 38,567 to all 5 champions. I don't care how tough you are. Your biggest threats are immediately eliminated leaving just the tanks with almost no hp left. (Yes a rammus with 150 armor-in Dominion its less than normal since they aren't really tanking- with 3k hp would be:
3,000*2.5= 7,500 theoretical hp. This means with all of his buddies he's dead, but if hes with only 2 then hes low enough to take down in 2-3 more shots. Now of course this bring in the worry of thornmail completely raping, but remember you will have 15% lifesteal going off of the 7k damage not the 2,571 the thornmail is going off of. This means you will out-gain the damage done.

I'm not one to talk long, and I'm too lazy to make this look nice, but in short Tiamat should be in an AD Twitch's build. I dare anyone to deal as much damage in the same time to 5 champions as Twitch. Remember this only works if you have your ultimate on. An aoe of an aoe means death to your enemies. (like my rhyme?)

Oh and as a closing thought...What happens when there are minions put into the equation...If in a normal match you come across a stack of minions and 5 champions all in one spot (The rarity of them being close enough together is unimaginable), then you technically have almost infinite damage for one shot. Hopefully I can post a video of this phenomenon soon.
Just to cure your curiosity especially after reading this wall of text:
734.6*.5*15+734.6= 6,244 damage in one shot with 15 creeps/champs in one spot. A scenario you might ask? Your opponents pushing a tower, and you line up perfectly to hit them all. If you did it right, and with some luck, you may have scored yourself a one-shot penta-kill. Of course this is very rare, but its fun to think of the possibilities.