Most guides on Mobafire focus on ranked games, which are for people who generally have no more need of these guides other than to compare to their own playstyles. Those who never played a champion before and seeking a guide for NG normally find them, and though the guides still work, they work better for experienced players.
So this is a guide wrote for general NG players who are looking for fun, for a high risk high reward build can see Nocturne at his full snowballing potential.
P.S. for those who are wondering if this build will work with an enemy team that isn't full of less competent players, I don't know my exact elo, but my victory is about 50~60 more than my defeats since level 30, never falling below 40.
This guide focuses on itemization and playstyle.

Nocturne is the fear of all laners, being able to dash with his ult straight through warded areas and deal tons of damage and easily chase down fleeing enemies. He has an absolute amazing ganking ability and can duel pretty well while being able to clear jungle with lightening speed when built offensive.
People often build him tanky, and while that is more viable in high elo games (or so they claim, i wouldn't know since I'm an NG player who has no ambition towards Ranked whatsoever), a pure offensive nocturne fits the nightmare theme more aptly.

Pros: great gank, good dueling ability, fast jungle clear and snowballs hard.
Cons: if behind, wouldn't be that much of an asset unless you coordinate with your teammates well, and not much of a help in teamfights.

At the time when you finish wriggle's lantern and is half way to your first brutalizer, stop a minute and ask yourself. Am I way ahead of the enemies? If you actually participated in 5 or so kills at 12 minutes, then by all means, start snowballing like a boss with this build.

Why not go straight IE PD PD BT Warmogs if you're going for pure offensive?
Because your early ganks will suffer for that, totally decimating your chance to actually reach full build. (the warmogs is there for when the enemy teams starts to really hate you, as inevitably they will as you go 20/3/8 or something like that on the 40 minutes mark, and being more fed than all your ADC to boot)

Ok. Back to this build. When you decided to build pure offense, start with brutalizer. It significantly increase your damage, while also giving a 10% CDR which will help you gank more, or land that second Q. It also gives the best DPS when chasing, as you can only spare that much time attacking instead of chasing or your prey will escape. So that eliminates AS (unless all three lanes have hard CC.)

Then on to BC and Ghostblade. Normally, BC is better as it gives more damage and a little bit of health, but if you're really really sure you won't die, go ahead and build Ghostblade as you can deal more damage and spend less time chasing (faster MS) with the active on. Then build the other one.

Why Zephyr?
Overall, an impressive addition to your DPS with awesome "side effects"
With the build above, you are lacking tenacity which when bought in Mercury Treads slows down your jungling and weakens your ganks early and now AS gives more DPS than AD, so zephyr it is. With you going pure offensive up till now, you are likely to be CC'ed in fights, and that time means much Damage Potential lost, hence the tenacity is needed (and it also gives a chance of an untimely SHUTDOWN). It also has a little AD (you have about 280 now on Q's trail) which is nice, but many people doesn't realize how much the 10% MS and 10% CDR matters on a nocturne with 20% CDR already from BC and Ghostblade. This will drastically increase your chasing ability as you can land a second Q soon after the first and has a permanent 10% MS boost. Also, as your CDR gets to 30% (and with the 10% elixir to 40%), your ult will be up whenever you need it. This is the stage when you're presence on the team itself will become a huge pressure to the enemy team, as every time they think your ult's on CD, you will sweep in and KS the lot of them.

When and Who to commit.

Top and Mid is easier to gank pre 6 due to the lack of wards.
Try to have red when you gank, unless the target isn't at full health (which somehow is mostly the case...dunno why my teammates doesn't harass often)
Remember, try not to tower dive if you have exhaust instead of flash.

Top: the best ATM machine is a lane bully in need to dominate the lane (i.e. Pantheon). They will constantly try to initiate on your teammate and that's when you go in. Make sure your Q hits, and then go in and immediately E the enemy. He will most likely flash away even if he is near the tower when the fear will go off if he doesn't flash. Great success. Camp at the brush below and wait for the guaranteed kill. Exhaust if your teammate might die, or the enemy has an MS boost or dash.

Mid: as soon as he goes past the mid point of the lane, Q in and start chasing. E at the first possible moment and watch him burn that flash. Come back in again as soon as possible (i.e. wait in the brush for your CD and then leap right back in.) Burn exhaust if the enemy might escape to the turret.

Bot: Only enter if there is already a fight going on or they are way overextended. Try to cut them off from behind, and like before, burn E as soon as possible though in this case, you probable can still fear them even if they flashed as you will be behind them in the first place. Exhaust the one you are trying to kill if they survived your fear and still hasn't burned flash, and you will still net a kill for yourself. Waste it on the SUP if the exhaust determines a kill or an escape. $(4)300 is still $(4)300 whether it's from a support or ADC. You are trying to snowball!

Time Management
You don't need the xp and gold from jungle creeps that desperately to clear them the second they respawn. Only go for the buffs (yours and the enemies as you can mostly escape even if you will lose a duel due to your Q and if necessary, R to a laner and gank XD) unless the creeps are on the way from one gank to the next. NEVER go towards a jungle camp if it is out of your way. Prioritize ganks over farming. This is your job as a jungle. Kill champions, or waste time trying, as this will give your laners a distinct advantage that will turn into assists for them and kills for you. If you KS, then you benefit all three lanes. If they got the kill, they benefit only a single lane (albeit to a greater effect. still, 3 lane small win is better than 1 lane great victory...yes I know my grammar's broken, but try not to notice that). To sumerize, gank>counter jungleing>farm. Get it?

Who to Gank
The fed, the farmed and the jungler.
As you gank mid or top, you come will realize a distinct advantage over the opponent. You go 2 on 1. So no matter how fed the enemy is (within reasonable limits, of course), you can normally kill him. So deny them the chance to snowball themselves by killing the lane where your team are the most likely to lose. Top as they most likely will be unable to burst down any of you with your exhaust up, and mid because the mid champions are generally squishy enough that you will be able to take him out even if your teammate is bursted down.
Bot, well, being an assassin with exhaust and a fear, you will not be likely to lose a duel with the ADC even if he's fed, and if you time your gank so that the support's peel is on CD or he's too far away, it's an almost guaranteed kill, as you can easily chase the ADC down before he gets to the turret.
As for the jungler, because you are the one invading, you will know the fight coming and the enemy will not (wards people, and elixirs). So you can easily catch them on 70% health and win a fight easily with your god-awesome dueling ability. Just remember to catch them where the laners won't be in time to kill you after the fight.

Unless, of course, the reason they haven't surrendered by now isn't due to their having a chance to win. (yes there is always the chance, I meant that they doesn't have much chance to win even a 5V3 teamfight). Then, charge in whenever your teammates are near.

Ok. Now, if they haven't surrendered by now, most likely that if they take you out of the picture, they have on average a distinct advantage over your other teammates whether in team composition or in gold. Now, you will not be able to charge right in and run out after a few kills. Wait for your teammates. Only initiate if you find a lone enemy that you can kill before the enemy team react after the time you bought with your ult. Otherwise, just be the average offtank with your one and only defensive item and wait for the opportunity to assassinate the AP or ADC.

No pictures WTF?
No I just wanted to do a rough guide to point out the snowballing potential of Nocturne in NG