This is my second guide here. I play Nocturne as my main jungler, because he is very strong at it. With this build I usually win solo queue games. I don't use the jungle path that I mentoin here, but it's almost the same.

I use Greater Mark of Desolation for late game power and jungle speed, s for jungling sustain, s to survive late game burst, because i got only one magic resist item on me and , because they are very powerful.

I want to have atleast 10 points in utility, so that i get 30% more neutral buff durations, like lizard and golem buffs. I also want 11 in offensive so that i get armor pen from there. I need 6 armor from defensive tree to clear jungle safely. So i use 11/3/16 for this purpose.

I rush gold per 10 items and wriggle's lantern, because they are really useful on almost all junglers. After that i get ghostblade, because nocturne is meant to be an anti carry. I get boots after, and tank items. After these items I am tanky enough to initiate fights.

Heart of Gold Gives early health and gp10, I get it if I can before 13 min mark.
Avarice Blade Gives critical chance for boosting jungle speed, and gives gold per 10.
Youmuu's Ghostblade Active is very useful, and upgrading item is good also.
Mercury's Treads Alternatively Berserker's Greaves, but usually enemy has 3+ CC abilities. This reduces their effect.
Frozen Mallet I prefer this instead of Warmog's Armor, because is an assassin and needs the passive from Frozen Mallet.

Nocturne can go blue golem route for early ganks/level 2 gank, but i prefer the route where i can counter jungle freely. This saves gold on potions and better sustain in jungle.

Route
Wolves with Q, attacks and Q again.
Wraiths by smiting big wraith, Q the small ones and attack them twice.
Golems by using two Qs on them and attacks.
Go base and buy Cloth armor and 1 potion.
Do blue golem by using Q on all monsters twice and attacking golem.
Do wolves same way as before.
Do wraiths without smite, focus the big wraith and try to hit all 4 with Q by being a bit off straight (Can't explain, sorry).
Do golems same way as before.
Do lizard same way as golem.
Start ganking and jungling if there is no gank chance. Counter jungle the opposite side of the jungle where the enemy jungler is if you can't assist the lane. This means that if the jungler is bottom and you are doing blue(Blue side), go to their lizard side and clear it.
Make sure that the enemy laners fear you.

Max Duskbringer, because you will be using this ability the most, and it scales best. Take Shroud of Darkness at level two, because its passive will speed up your jungle, and leave it there for the rest of time. I max Unspeakable Horror second, because fear duration is really nice to have.

If you want to do blue golem first, I think that you should get W first for the attack speed that it provides from the passive.

I take Smite on Nocturne, because every jungler needs it for securing buff and big monsters. The second summoner spell is more optional. I prefer to take Flash, because it allows me to get out of almost every situation. I have seen people take Exhaust too, but it only improves ganks, 1v1ing and shutting down the enemy carry. Use what you prefer.

Try to keep jungling, which is explained in jungle chapter. Try to make sure, that the enemy doesn't get dragon and ward the lanes who don't have gold for wards. If you see that no one is in lane, go there to push it, so that enemy xp is denied and you get gold for it. Do it only if the lane is close to you.

Small team fights and ganks start happening, try to lead your team for ganks. You have the strongest ganking ability in this game, because the enemy can't see where the gank happens early enough. After ganks try to push lane and after team fights try to get the dragon (If you win the fights, of course). Dragon gives a lot of gold (like 190 to every member in the team), so it's the most important thing for you to control after baron, which shouldn't happen yet unless you win the team fights totally. If your team has another wriggle's lantern tanky champion, you can try to ninja baron.

Nocturne is not the strongest here, but he is still quite strong. Enemy should fear you, so try to fight in the jungle. You can split push to their base, if your team can hold them 4v5 near tower. While pushing tower, press Duskbringer and Youmuu's Ghostblade for damage and killing power. If enemy comes, you should either retreat if he is strong or the enemies are coming, or kill him if he is weak.

In team fights you should initiate fight on their carry with your ultimate, if you see her out of position. Try to fear their carry and hit Duskbringer on her. If the enemy tries to stun you, press Shroud of Darkness before it. This can be hard to do, but better to try it. If you get focused down hard, try to back from the fight with flash if you can survive.

I made this guide to help people to play Nocturne. He can be very fun to play, if you like his type of champions. He has amazing skills, but ultimate can be hard to use in the right spot. It no longer grants assists for preventing vision, so don't use it globally, unless it saves someone.