Firstly, welcome to my ASAD champion guide,
I have used this site for quite some time now for builds for a number of different champions, including build for Nocturne and Shaco, however none of the builds I could find were just right, so I decided to modify one. After a lot of playing around with different ideas I came upon this set up. It has been incredibly successful for me, and for a number of other players I have given this build to. As a number of those summoners have suggested I have decided to place my guide up for everyone to see.

Some terms you need to know to understand my guide:
Q means Duskbringer
W means Shroud of darkness
E means Unspeakable horror
R means Paranoia

Pros:
*Great harass, with your Q you can skill shot most enemy champions from a safe distance
*Great farming, your Q does tons of damage to minions plus your passive allows you to do massive damage to multiple minions at once
*Fear, you have a fear (E) 'nuff said
*your safe, with practice you will be able to block almost any skill shot with W
*escape artist, your Q allows for faster travel, and if enemies are following closely your R makes for a great fear to divert the chase
*Your AD, so a silence is little problem
*you can solo dragon at level 9 providing your at least at zeal item wise in 3v3 and level 10 in 5v5
*you can jungle

Cons:
*Your squishy, but then again that is what the life steal is for
*this build is all offense, so if possible stay behind or with a tank

summoners wrath is for surge so if you decided not to go with surge then you don't need this.
For the most part the masteries are all attack damage, attack speed and life steal.
The cool down reduction is quite helpful when you have to chase someone or run.
The magic pen is there simply because it provides a little boost for your E and one mastery point in another section isn't going to provide a lot of help.
Except of course in the case of summoners insight, which gives you a large bonus of mana when you use clarity, if you decided to ditch clarity then ditch this for enhanced recall or demolitionist for some extra turret destruction.

Now everyone looking at this guide is saying surge + clarity what a noob, however, these are not set in stone you can change them, but even still I do believe these are the best choices.

MY CHOICES

Clarity because early game the mana allows you to stay in lane longer and keep farming, it also allows you to spend some mana using your spells on dragon or buffs if you want to kill them quickly then get back into lane.

Surge is simply because you are an AS AD champion meaning an attack speed bonus is awesome, not only that but it provides a little bit of AP for your E, not a lot but still its an added bonus.

GOOD CHOICES

Teleport - helps you stay in lane and get over to other lanes for a surprise R
Flash - was originally my D spell it was amazing, but its been nerfed and i believe surge is much better, however this does provide many escape options, from a quick distancing to wall flashing.

NO, JUST NO
Ghost - you don't need this, if you cant escape with Q and E ghost would have done very little
Ignite - You don't need this, if someone is escaping with a sliver of health R them, if you already used your R then you overestimated yourself, don't let it happen again
Clairvoyance - No, no, no, no, no and once more for good measure no
exhaust - If you are in range to exhaust your in range to Q, E or R and if all of those are on cool down you are doing something seriously wrong

There are two basic parts to this:
Attack speed and Armour Pen

Attack speed allows you to get a lot more hits in before the enemy realizes they are done, and more hits also means more life steal.

Armour pen is for those times when you come up against a tank, it allows you to push through a good enough amount of their armor to do some damage.

Q this is your initiator, the first skill you add to and the first you max, does a hefty amount of damage on squishy champs and a good amount on tanks. It also leaves behind a trail on those champs allowing you to outrun them and avoid colliding with units. On the other hand if you are being chased you can send this the other way and use the move speed bonus and unit collision avoid to get away.

W this is your spell shield, it allows you to stop the first incoming spell, so no Nidalee harass, no giant frozen arrow from Ashe, no fear from Fiddle, no parley from Gankplank. But is that it NO, when you block one of these spells with this shield you increase the time you gain bonus attack speed from it.

E your fear, you can fear a champion for extra hits, simply to do more damage or gain more health from life steal, if your enemy decides to run you can fear them to screw with their escape. if and enemy is chasing you and has a good chance of reaching you, fear them to improve your escape odds.

R the reason you probably know about nocturne, the enemies screens go black, and they can see almost nothing allowing you to escape or pick off an enemy without them knowing, and tap it again and you can rush to enemies in an enormous range. useful for turret diving or just plain chasing down enemies.

I start this off with a vamp scepter because this allows you to stay in lane longer, you can heal yourself of minion especially with your passive as it will hit everything around you and you can still life steal that.

I build this into a wriggles lantern right away for the bonus life steal, some extra damage and the ability to place a ward, since you are squishy this allows you to watch for ganks.

I then build into berserker's grieves for the movement speed bonus and the attack speed bonus, simply because this allows you to run, chase and attack faster.

Next up is the phantom dancer, because it gives massive amounts of attack speed, a much larger crit chance and even more movement speed.

Then comes the black cleaver, this is one of the most important items, it gives more attack speed, more damage and most importantly of all each consecutive hit on an enemy allows you to deal increasing amounts of damage.

Next is the bloodthirster this gives tons of damage and tons of life steal this is a necessity if you think you already have enough life steal and you want more damage you are wrong, if you really NEED more damage trade this for your wriggles lantern.

Finally comes the infinity edge, a lot more damage, an increased crit rate and now your crits do a whopping 50% more damage than a normal crit.

All in all nocturne is amazing and a fed nocturne means game over for the other team, I hope you like the guide please give me suggestions on other items/runes/masteries or even summoner spells, be sure to include some reasons though, I will try out suggestions in game before making any changes to my guide.

most of all good luck have fun summoners.