This build is pretty much finished, there might be some things I forgot but the core is here, it is also the first time I publish a build online so constructive criticism is welcome.

I decided to upload this build since I appear to be the only one using it, even though it works very well. (haven't played ranked though)

The idea of this build is that you can ignore the armour your opponents have, you'll end up ignoring a little more than 110 armour, which is less than most non full tank characters have. The reason I do not use % armour penetration is that it is only applied after reduction and flat penetration which causes it to always keep an enemy's armour high (at least 25% reduction)

This build will not give you as much Attack Damage as other builds, but it compensates by ignoring enemy armour and will end up doing more damage (usually) This build Fails if enemies build a lot of armour, about 260 enemy armour will break this build, but probably break their char too so just ignore that target and focus someone else. in the extremely rare event that all enemies get more than 260 armour. Congratulations you are now so scary that they broke all their characters in order to stop you, get % penetration and just build regular AD (you still won't really hurt their characters though). but this never happens.

Some information might be mentioned multipile times in this guide, that usually means it's important.

I recommend red armour pen runes, armour pen quintessences, armour seals and magic resist glyphs
The defense will really help you at low level and the armour pen is very important for this build

Most of these masteries are aimed at increasing your damage, butcher is mostly personal preference, it makes you a little better at pushing, just like demolitionist. most of the masteries are really the only way to go if you want the last talent, I have been thinking about swapping out the 10% armour penetration talent for some extra armour, magic resist, or scrapping those two too and taking some extra movement speed.

I find vampiric scepter a great early game item as it will give you some sustain (this is a laning build in case you didn't notice)
One of the most imporant items of this build is The black cleaver as it will give you 45 armour penetration (I know you technically lower it and you need 3 hits to apply it but you will have that armour reduced in no time)

There isn't really a build order, it all depends on how fed you are, I recommend buying zeal after boots of speed but before berserkesr greaves because it gives more movement speed and other stats for just a little bit more gold. You should judge which items to buy depending on the situation, lack sustain? get stark's, want more damage/lacking gold? get brutalizer and averice blade (don't upgrade to ghostblade yet if you lack gold).

Don't be too afraid to trade kills, just make sure you're not giving them more money than they give you (for example your team gets a kill and an assist and they get a kill, this situation is worthwhile) also be careful not to feed the enemy carries.

If you die a lot I recommend getting guardian angel (You can always sell it later for infinity edge if you want, though Guardian angel is viable too) it really helps with kills.

Team work is very important for nocturne, especially in early game. You can usually make at least one early kill if you work together with your ally, common lanepartners for me are Taric, Galio and Ryze (due to my friends their preferences. You need to coordinate quite well. There is a fine line between killing and being killed. If you don't get early kills, be sure to farm a lot.

If you gank other lanes, be sure to tell your allies and ping the target when you are about to attack. Paranoria can be a great help when ganking.

Always remember, assists give gold too, always try to keep cool when other people take what you view to be "your" kill.

Farming is very important, it gives a lot of gold and it pushes the lane.

Nocturne is generally a great champion, here are some pros and cons:

Pros:
Great ganking potential with paranoria and duskbringer
Duskbringer can be used for a lot of situations, it is great when running away, when chasing, when harassing and when farming.

Umbra blades passive greatly increases your farming capability.

your shroud of darkness skill is great against all casters, more about this in the Unique skills section

Nocturne is quite tanky

Paranoria can make people freak out.

Cons:

It is not unusual to be focused
If you do not get kills/farm enough you will be almost useless
you do not have a lot of CC

In this section I will handle each of nocturnes skills one by one.

First of all the passive: Umbra blades, there is not a lot to say about this skill that isn't in the rest of the guide, but I'll recap: It greatly improves your farming ability, and restores your health.

Then you have Duskbringer: This is a really nice spell for almost all situations, it can be used to increase your movement speed while running away, it can be used to harass and to chase. It is also a great spell for farming.

Then we get Shroud of Darkness: This spell may require some practice to use correctly: It is a great spell against most casters, enemies are usually predictable, for example Ryze will usually start with his snare, use w to ignore it and increase your attack speed, then proceed to burst him down. Annie will usually stun, lux and morgana might snare etc.
Gangplank is also much easier to defeat due to this spell, just use it when he fires his gun and burst him down.

If you are up against a Karthus this spell deserves some special attention, Save this spell for emergencies and when you see Karthus use his ultimate don't immediately use this. It won't last long enough. Instead wait one or two seconds to use it. It will save your live. A lot.

Then there is of course the passive effect, which is also quite nice. It increases your attack speed passively and when you absorb a spell.

Then there is unspeakable horror, this spell is nice to use when you are chasing, chased, or about to enter a fight. Be sure to stay close to the enemy when using it (or not if you are being chased) as it will break on range. Note that the fear has a two second buildup time.

Then we get to the last spell, Paranoria. This is (together with rocket grab) my favourite spell the game, enemies freak out when everything goes black and you can launch yourself to a target enemy. Note that this ability can sometimes be used to jump to safety if you are being chased. You will need to be careful if you are using this to flee though.

I am not sure at which levels you can pick everything, basically first take duskbringer, then Shroud of darkness and then take unspeakable horror. When you have all three, just max duskbringer first, then max Shroud of darkness and then max Unspeakable horror. Put points in paranoria whenever it's possible.