Hai and welcome to my very first build, which is on twitch the plague.
I think twitch's nickname, the plague is extremely fitting. He is a big pain in the butt to play against if he's played well!
So now im going to show you how i play Twitch!
I just want to point out that this is ofc a discussable guide and i therefore gladly read comments and good tips. I also answer questions that you might have!

The city-state of Zaun is a twisted realm of science, mercantilism, and magic run amok. While the young science of techmaturgy does much to advance civilization, its research and use in Zaun pushes the boundaries of morality on a daily basis. Zaun, choked with pollution from the countless factories and laboratories that spew waste into the environment, suffers greatly. As polluted as Zaun is above-ground, its subterranean levels are far worse. All of Zaun's runoff waste pooled together in its sewers, mixing into a toxic and mysterious concoction. A single plague rat rose from this alchemical nightmare - Twitch. Unlike his lesser evolved cousins, Twitch became as sentient as anyone else... provided they were as insane as he. A seeming byproduct of the evolutionary brew that spawned him was maniacal insanity, albeit in a functional form.

Twitch came to the League of Legends to represent himself ''as the only one of his kind'' on the Fields of Justice. Since then, he has found a generous benefactor in the city that gave him his evolutionary sentience - Zaun. Twitch is one of Zaun's premier champions in the League, and as such he fights for Zaun when political disputes between Valoran's city-states arise. Twitch has publically stated that his goal is to gain enough influence and materiel from his work in the League to try to recreate the process by which he gained his sentience. While many challenges lie ahead in his quest - one of the largest being his own insanity - were he to succeed, it would mean a bold and brazen step forward in the evolution of all of Runeterra. Zaun has pledged its support for Twitch in his quest, though it remains to be seen what Zaun's ulterior motives are in the matter.

"The existence of Twitch proves that anything is possible on Runeterra." -Heimerdinger

QUICK GUIDE:
Use to sneak up behind opponents, get as many as possible stacks, pop and if they run, then Expunge for maxing you're damage outcome. Remember and Expunge works really good together when fighting in team fights!


GOING INTO DETAIL:

Deadly Venom - Twitch's attacks infect the target with Deadly Venom, which deals 2,5/5/7,5/10 true damage each second, stacks up to 6 times, and lasts up to 6 seconds.

Ill start off with Twitch's passive called Deadly Venom. It's really useful especially in early game. Lets say that you hit a champ with 2 basic attacks really early, then deadly venom would inflict 30 damage to the target over 6 seconds. It may not sound as much but it does matter alot. So early on try to lay in some hits, a champ on half HP is more low profile and is not likely to open a fight!
Also youre stacks of deadly venom inflicts the stats of Expunge and Debilitating poison so it's always good to have some stacks!


Ambush - Twitch hides in the shadows, waiting for a nearby enemy. If he attacks, he will gain bonus attack speed.
After 1.25 seconds, Twitch becomes stealthed for 10/20/30/40/50 seconds. If Twitch attacks a unit while Stealthed, he gains 30/40/50/60/70% attack speed for up to 10 seconds, depending on how long he was stealthed before attacking.
Cost
60 Mana
Ambush is a really useful spell, especially for 2v2 lane fights! But always remember to stealth out of opponents vision else he will be aware and most likely run for a tower.
So why is this useful in team fights? You run up behind a enemy champ and open the fight with you're extra attack speed, if you're lucky the enemy will run away from you and towards you're team mate which is standing either near you're tower or in the bushes. you both start to cut down his HP which is gone in seconds! then you pop Expunge and get the kill! then the remaining opponent is alone and has little to no chance against you and you're team mate!
Also Ambush makes it possible to escape the most insane situations! when you get ganked or jumped just pop that Ambush and run! you'll have a good chance of escaping!


Debilitating Poison - Infects all nearby champions with Debilitating Poison, slowing their movement speed.
Slows nearby enemy champions' move speed by 30%, plus 6% per Deadly Venom they have.
2/2.6/3.2/3.8/4.4 second duration.
Cost
80 Mana
Range
1200
This spell is one of those "why do i need it?" spells, but it comes in handy when chasing and ganking. so basically you don't really need more than one point in it in early game. Twitch has much more useful spells to use his points on! But never the less it got me some nice kills on chases, and the fact that it inflicts all targets in visible range is good in team fights!
So use it when necessary but don't waste points on it from the start.

Expunge
Expunge - All nearby enemies secrete Twitch's toxic venoms from their bodies, dealing damage for each stack.
Deals 30/60/90/120/150 (+1) magic damage to each nearby poisoned enemy, plus 20/30/40/50/60 additional damage per stack of Deadly Venom applied to them.
Cost
90 Mana
Range
1200
This is my favorite spell of all time! this is useful in so many ways and in the whole game not just in the start or near finished. it's also really simple, you do damage per stack you have on you're enemy so just get those six stacks and pop this! most champs are low HP after that! if a low HP champ runs make sure to have some stacks on him, he's dead right when he thinks he got away!:) Remember to use this in blinds! if a champ runs into the woods or bushes he still has you're stacks and will take damage from expunge. This works great with Twitch's Ultimate, Spray and pray too! pop SaP, get some stacks on multiple enemies and then use Expunge! it turns the whole situation around and now you lead the fight!
Twitch's main damage spell and a pain to all opponents!


Spray and Pray - Twitch closes his eyes and fires his crossbow, spraying powerful piercing arrows ahead of him. Twitch's attacks become long range, passing through units, and he gains 15/25/35 Attack Damage.
Lasts 12 seconds or 5/6/7 attacks.
Cost
150 Mana
Range
1200
Twitch's Ult, good for hitting more than one enemy and getting stacks on them both! remember Expunge hits multiple opponents! good for chasing too since it expands Twitch's range to his vision range. So ambush, get some stacks, if they run pop this and Debilitating Poison and when they are low health use Expunge!

Highly recommended:
Exhaust works extremely well with Ambush and it helps alot getting the required stacks of venom for Expunge!

Ignite is also really nice and works well with Twitch's poison!
Also usefull:
flash is kinda useful for running but i don't recommend it. but if you're anxious all the time, Flash is ideal for running. It also works good with ambush, if you get jumped you just flash away from them and pop ambush then you're gone!

Ghost is also good for running. kinda explains itself ;)

Teleport is also useful, and again, it explains itself.

Heal, if you're worried that you might die, heal is good to have with you! But i don't recommend it, since i think offensive Summoner spells are more usefull.

This is going to be pretty short and i'm not going into detail with all the masteries, but if you guys would have done them differently just let me know! I'm sure there's people out there who would not agree with me at this at all:)
But basically i just focused on the, to me, useful-looking masteries in the offence tap, things like attack damage, life steal, crit damage and so on. Ive also placed a point in Summoner's wrath which boosts exhaust and ignite. This point would ofc be place differently if you're not choosing exhaust or ignite as you're summoner spells. But remember Summoners wrath also affects ghost so if you have ignite and ghost or exhaust and ghost it would still be a good idea to have you're point placed there.
Then Ive chosen to place 9 points in the defense tap. This is due to 2 reasons. First off, Theres not really more you need in offence so it's better to place em elsewhere. Second reason is that Twitch is really a wanted target in team fights and he's also squishy! so Ive placed my points in flat armor, magic resistance and health and some health per level resulting in giving him just a little more health:)

Okay here's where the sh*t happens!
Recommended:
First off get you're Doran's Blade! it gives you a highly suitable health bonus and also some attack damage!

Then you grab you're minor speed boost if you have recalled before you can afford Berserker's Greaves.

Then get you're Greaves, that gives you a good attack speed bonus, making Twitch get his venom stacks faster!

The scepter give's you 12% lifesteal which i recommend if you're game is going well, if not, get you're Phantom Dancer before you're Vampiric Scepter. you'll need that more :)

Zeal gives a good attack speed bonus, helping stacking, plus crit chance and movement speed which is always welcome;)

this is required for the Phantom Dancer so just get it:) the crit chance is also good to have.

This is where Twitch is becoming a dangerous opponent for you're enemy's! Phantom dancer gives you 55 attack speed and 30 crit chance resulting in you firing away arrows quickly and making crits pretty often!

Gives you a nice damage boost. if you need a quick damage increase get the Pickaxe before you're BF Sword. i may not give as much AD but 25+ is a nice portion anyway.

AD boost.

Sh*t gets real here! With Infinity Edge Twitch EATS HEALTH POINTS with his basic attacks! and with a Crit chance on 60% and a crit damage on 250% his crits will hit nearly 500!

Now i usually build a Blood Thirster because you already have the vampiric scepter you'll only need this and then the recipe cost:)

Gives a nice cocktail of lifesteal and AD:)

You might think "BF sword again?" yes! Twitch already at this point have the needed attack speed and mainly need to increase the damage outcome per hit.

Gives AD and attack speed:) basically just makes you're damage happen faster and quicker! also it's unique reduces enemy's armor by 15 per hit, 3 stacks! -45 armor really has something to say:)

This is a discussable item and i only recommend it if you tend to die in situations where you think you could have gotten away with a little more health. Banshee's Viel also blocks one spell every 45 seconds, which is useful.

You could also get one more Cleaver instead of Banshee's Viel. If you want damage not health points;).

At this point you don't need it anymore, just sell it :)