Nocturne is a jack of all trades, he can jungle, carry, or lane with a partner. For this build he is a carry. Your probably asking why carry, well its because of his huge ganking potential. If he is higher than the people around him you are almost guaranteed a kill or maybe two. You will ambush people who pushed up just a bit to much or change a 1v2 into a 2v2.

Try it out before you rate. This is my personal build and you can change it how you like.

Pros
Good initiator
Good Ambusher
Great Duelist (1 v 1s)
Insane Chaser and Runner
Nice spell shield

Cons
Bad at teamfights
Need good teamwork
Slightly Squishy

Runes are your choice, mix and match to see what you like. I usually go with attack penetration marks and attack speed glyphs for offence, and Armor seals and Health Quints for survivability.

Duskbringer is your bread and butter, the thing that makes Nocturne destructive. You can use it to engage when your ultimate is down, and chase enemies. When your ultimate is up your going to make people run and this is how you deal with runners. If you get ganked then just place it towards your tower and run through it, the movement speed WILL save your life. The increased attack damage and movement speed WILL help get kills.Shroud of Darkness is a powerful tool. Completely nullify one spell and turn it into a buff. It's great for early game when they try to harass you if you get the timing down, then you'll surprise your opponent and even take out a nice chunk of health or kill them. The main reason for it is because of the passive attack speed which lets you output that much more damage.Unspeakable Horror is good for finishing low health champions. It will be your first ability used after you initiate, and can keep you from losing precious Hp. Keep enemies in the fight that much longer.Paranoia is the initiator, help gank in other lanes, Avenge your teammates by killing those low health champions who killed them, help ganked teammates out and turning the tables, run past the team and target the squishies hiding behind there team or the ones that try to run away.

The skill sequence is get your ultimate when you can, max out duskbringer, then your shroud, and lastly your unspeakable horror. Make sure you get at least one of each spell by level 6.
R < Q < W < E

In Teamfights
Who is the major threat? Once you figure that out let your team initiate while your hiding in the shadowes, when they do ultimate in, then use unspeakable horror to keep them from running. When the fear comes up, use your duskbringer to make it impossible to run away. Now is the time to activate your shroud of darkness to keep them from slowing you or doing unnecessary damage from enemies. Keep doing this until each of the champions are dead. Make sure you target the ones farther back once the main threat is dead, that way when they see they're losing you will kill the ones closest to their base and sandwich them and watch the kills rack up.
One on Ones
Here is your chance to shine. Let them gain a false sense of security by letting them push. Once the get close enough auto attack and put up your shroud of darkness, then activate your unspeakable horror and activate your duskbringer. You should have the advantage now, and when they see their health drain they will try to run. Use your ult and duskbringer should be back up, use it and finish them off.
Ganking Outer lanes
Tell your team to let them push, when they do ultimate in on the one hiding further back and tell your team to initate. Use your shroud, then the fear and after that use your duskbringer to chase and finish. If you did it right then you'll get a kill if you did it amazingly then get you'll clear out the lane.
Saving Teammates
This is easy, get close enough to ultimate in and use your fear, if there are more than 2 people use your shroud and run away, if is only one person then use the ambushing technique. If there are 2 people then that will depend on what your teammate does, if they stay then attack if not then run away.
Preventing ganks
If you can use ghost to runaway, if it's on cooldown then use your duskbringer towards your tower and move back. Use your shroud when they get close and it could stop a slow or stun.

Start off with a Vampiric Sceptar for the extra survivablity. Stay in the lane until you are level 6 and have enough for Berserker's Greaves for the attack speed and movement speed boost. Grab a Bloodthirster if possible or a BF sword next for the extra damage. Your gonna need some more attack speed so grab a phantom dancer to make your passive more noticable. After grab an infinity edge or a banshee's veil depending on how well your doing. One spell block will help if your getting focused, and the infinity edge will help with the extra damage. Get the one you didn't get next. The last item is a choice based item. I like getting another phantom dancer, but a black clever, or last whisper are also viable.