This is a Laning Nocturne build focusing on harass and careful play to maintain lane strength, turning into a long reaching assassin/chaser. The focus of the build is to move in kill then LEAVE, this build is not particularly strong at team fighting, coming into it's own when ganking early to mid game using the Ultimate and surprise to kill.

the runes I choose are rather obvious choices in my opinion, Armor Penetration to hit harder in the early game, coupled with a little extra mana regeneration to keep you from constantly have to sit in the back for lack of mana, not that you should have to anyway if you are casting conservatively.
The Cooldown reduction is also an obvious choice to me, giving a boost to be able to get off that last Unspeakable Horror, or Nightbringer to net the kill.

Quints are a different story though, you can go one of three ways:
Flat HP and Flat Regen make for a much stronger early lane phase, however there are 2 other ways to go that both could server quite well.
Armor penetration quints iwll make your early hits MUCH stronger and make minion farming a formality and a breeze,
The last option is to go with movement speed quints to be always just a little faster than the opposition and you usually will be quite a bit faster than the enemy can be. but they stand as a good idea none the less.

Masteries are rather standard for a melee carry. attack speed, attack damage, crit damage and Havoc going right up the tree. Utility again is focused on getting to one point in meditation to fix the mana throughout the game giving you an extra 3 mana/5. this SHOULD be enough so you are always at close to 50% mana in lane phase, throughout lane phase. otherwise the masteries are focued on making him hit harder.

Items on Nocturne make this champ as powerful as he is. The deal with him is that in most cases stacking damage is less effective than building to reduce the enemies armor, and a lot of people don't build as much defense as they should. this build will easily land opponents in the Negative Armor zone if they don't build armor Items, and that's what we are aiming for. So the early lane phase is easily weaving in and out of bushes harassing with Nightbringer and using the trail to farm on as it gives a boost to damage. this damage will only become more significant as you work into boots of speed with priority and brutaliser, which will then become Youmuu's ghostblade. This is the point where you should be around level 6 and probably have your first kill.
This makes your ganking go through the roof from here on, picking up regeneration and lifesteal from the emblem of valor to keep you in the field longer. This is then followed by some real damage to take advantage of that tiny armor they have, the Big Friendly Sword. This item is followed by some defense, either Negatron or Chainmail whichever type of damage is giving you more trouble. this Item can build into whatever you want, Sunfire cape and Banshee's veil are both good choices depending on which direction you go. another option oft overlooked is to get a Guardian Angel. After getting to this point the Black Cleaver and Stark's Fervor are both optional Items suggested to keep in staying with the theme of armor reduction and penetration.
The Sixth Item has been purposely left blank, I usually keep a MESS of wards around becausue nothing helps paranoia like being able to actually see the enemy. but if you must get a static Item I reccomend either The Bloodthirster or Madred's Bloodrazor, to be built as you need them. sometimes Emblum can be swapped for a BT but keeping it stacked can be a problem. So in general the Emblem is better. Madred's should be built as soon as high HP targets become a problem, interrupting whatever else you are doing to get it.

This section will be rather short section compared to the above, since this build is focused on melee damage the nightbringer's passive damage boost figures in highly so is maxed first. then taking your ult whenever it's availible and alternating between Unspeakable horror and Shroud of Darkness to give you an ever increasing ability to chase successfully. REMEMBER Nocturne is NOT a front line soldier, he is not Sivir nor is he Tryndamere. he is TRUELY an Assassin. lurk around the edges, when the teamfight starts use paranoia to keep them from helping one another and fall upon their support and Shred them up with impunity before focusing down DPS.

In lane, it is imperative that you NOT spam your skills, use them strategically. you have a GREAT physical damage nuke in the form of Nightbringer, while standing on it your Umbra Blades makes farming a Breeze. your Umbra blades last hits Every minion in melee range. use this ability for 2 things, regaining health and taking wave for yourself, by about level 6-8 Nightbringer + Umbra Blade should kill at least all the Caster minons, but otherwise Just lasthit as you would with any champion, Don't try to use abilities to last hit, be threatening , unless they are a ranged carry you generally have nothing to fear from them. just Umbra the entire next wave once and regain 50-80 HP then hide in the bush and come out to last hit or Umbra blade agian, you shouldn't just auto-attack minions this makes it harder to last hit them for maximum gold, by 10 minutes you should have at LEAST enough gold to go back and get boots, if not boots and a longsword.

The pros and cons of this build should be somewhat obvious.
Pros: Strong ganking and amazing chasing ability even through jungle, Assists are fine too. you have a Large damage output and your skills legitimately hurt. you have a good amount of survivability with decent ability to regain HP should it fall low.

Cons: Not good at a stand up fight, can't initiate fights, cannot really solo lane with this unless it is 1v1. cannot spam abilities, cannot win an even fight with many other champions, requires a good sense of timing to make work in a teamfight.

So, to conclude, this build for Nocturne is less about being an anti-carry like Evelyn can be, or about being a good teamfighter/ganker like Shaco, but Truly embraces Nocturne as an Assassin. someone who swoops out of the darkness to kill and then fades away before it's even known he was there. using a combination of Armor reduction and penetration with High damage output items and careful use of skills to make a deadly warrior who is adept at Guerrilla Fighter specializing in hit and run tactics.

He is a bad dream, only you aren't too likely to wake up from this one until your nexus lies in ruins. Play carefully, and act like a demon of the dreams, an Eldritch Horror glanced but never showing himself in the open.