I'm going to introduce this guide with a tl;dr (too long, didn't read.) so that if you aren't sure this is your thing, you can check.

tl;dr: A nocturne build focused on heavy initiation with a few significant damage items to compliment your survivability, acting as a hybrid tank/assassin.

Now that that is out of the way, hello, and welcome to my Nocturne guide.

Nocturne has had a strange history with me. At times I have loved him, and at times he has fell to the side. At current, I would consider him my personal favourite jungler, due to his ability to initite hard and lead a team into victory. Nothing worse than a bad initiator in your games, at least this way, you can be the bad initiator so it doesn't feel as frustrating.

This build arose out of me building a more typical bruiser/dps nocturne with a Randiun's Omen, which made me think if that little niche of nocturne could be more of a thing. I feel it almost certainly can be.

Join me on my magical adventure through silver elo as I describe something that probably won't work that well past gold but who cares because it is fun. Really fun. Seriously.

Pros:

High survivability
Strong initiation potential
Innate spell-shield
Sustained damage output
Ability to deny vision of enemy team
Resistant to counter-initiation
Acts as both an assassin and a tank

Cons:

No game changing CC
No solid in fight gap-closer (ult doesn't really count)
No burst damage
Only moderate sustain
Sub-par pre-6 ganks

Passive: Umbra Blades

This passive adds AoE damage, with some sustain to Nocturne's kit. It helps his jungle clear and synergises well with attack speed, much like the rest of the kit (and the item build.)

Q: Duskbringer

This is many tools in one. Firstly it is a sticking tool, the bonus movement speed this gives, combined with removal of unit collision prevent all but the slipperiest get away without help. Additionally it gives bonus attack damage, which we will take advantage of through copious amounts of attack speed.

W: Shroud of Darkness

This acts as free attack speed, plus a spell shield, which then gives even more attack speed. This is crucial to a successful engagement as it allows you to shrug off one spell.

E: Unspeakable Horror

This is a single target fear, but you need to stick to the target for the fear to actually effect. The primary use of this will be as a minor CC to follow up with during a gank, and then a powerful single target CC to help remove the enemy ADC from a fight during team fights.

R: Paranoia

This is your means of engagement into the enemy team, your iconic ability, and the means of sewing chaos in the enemy team. It also massively increases ganking potential in the early game, and can be used to pick off low hp enemies.

We will be focusing Q first, followed by E. While maxing W second does give additional attack speed, I feel getting the two second fear earlier is more important than the extra 20% attack speed.

Runes:

Marks: Attack speed. They help in the early in the jungle, and they help in the late with devourer +1 on hit item.
Quints: Attack speed. See above.
Seals: Armour. New jungle hits like a ton of bricks, no other way to go about this.
Glyphs: MR/level. Magic damage is scarce in the jungle, and most magic damage will be in the late game. This makes MR/level the obvious choice.

Alternatives:

Marks, Quints: Attack damage.
Glyphs: Anything you want really, though I strongly reccomend MR/level.

Masteries:

The general theme is:

Firstly, tankiness is key; and more accurately, tankiness in armour and resists. Straight up HP has been avoided in most areas.

Secondly, small considerations to utility and clearing speed have been made, with a small foray into the offence and utility trees.

Thirdly, late game has been prioritised over early. This doesn't mean super late game, but a lot of the focus was on teamfighting, rather than gank potential. Keep this in mind when doing ganks.

A few things to note are:

Reduced effectiveness of slows: This prevents you from being kited, which is a good thing.

Increased AR + MR for every nearby enemy champion: This at full potential is 20 armour and 10 magic resist for diving into the enemy team. Also gets improved by the tier 1 mastery to increase bonus AR+MR.

Reduced damage from critical strikes: This helps soften the effect of late game ADCs, who will be your primary target for jumping straight in on. Being killed before your fear goes off is sad times.

Reduced damage from movement impaired units: With righteous glory (kind of) and randiuns as core items, this basically improves your overall survivability due to having a lot of slows in the start of the fight.

Summoner Spells:

Smite No other way, just literally no other way.
Flash Easy to use, familiar to most, and has arguably the most utility.
Ghost Alternative to flash, you trade utility and mobility for sticking power.

Please note the following:

Trinket choice is up to you, the player. Red is usually a good choice, yellow gives some extra vision. Taking blue can help snipe people with your ultimate. Whatever works for you works.

Starting items into early game is core.

Then go either Good/bad early or OK early, not both (it is simply a matter of order.)

THEN GET RANDUIN'S OMEN.

Following that choose one item from 'My god everything is dying.' Once that statement is true (the enemy team doing the dying.). Otherwise choose an item from 'My god I'm dying!', based on the situation.

At any time after RANDUIN'S OMEN, you may also consider an item from the bonus utility section, if you feel one of them would suit the situation better.

Also I'd like to highlight Turbo Chemtank's power potential even more than I already have. I may even consider it to be somewhat core alongside Randuin's Omen. However I felt it was not as good as a core, universal pick like Randuin's is and so I left it as a player choice.

In general, use your instincts and itemise based on the situation you are in. I have generalised for most situations, but a generalisation cannot be perfect for every situation. I believe in your judgement.

Good/bad early vs OK early explaination:

Good/bad:

If doing well, get an early Skirmisher's Sabre - Devourer, this will DRASTICALLY increase your killing potential and allow you to get stacks up early.

If not doing well (that is to say, you feel you cant do anything gank-wise at the current state of the game), dont despair, the Skirmisher's Sabre - Devourer first can still work, as you will not be ganking anyone if you are failing. Get Skirmisher's Sabre - Devourer and farm the jungle until you are back on your feet and have at least a warden's mail, if not randiuns, and shock the enemy team.

OK:

If your early isn't going so well (but you are still in the game and still have that ganking potential) you can delay the Skirmisher's Sabre - Devourer to get a Warden's Mail, which improves your tankiness to allow you to soak up some more hits. ITEMISING DEFENSIVELY ALLOWS YOU TO DUEL MORE EFFICIENTLY THAN ITEMISING MORE OFFENSIVELY - this is a CORE idea.

In summary:

Skirmisher's Sabre - Devourer first is good if you are forced to farm the jungle, or if you can afford to delay tankiness to get the early damage. This is why you get devourer first for good OR bad games.

When you aren't doing so good, you need a little extra tankiness to keep you in the ganking game, which is far better than dropping out of the ganking game when you really could avoid it.

RANDUIN'S OMEN IS MY WAIFU.

True story guys kappa.

My god everything is dying:

Good for you, now really drive it home with a damage item, pick ONE item that suits your tastes. Again, ONE ITEM, UNLESS YOU HAVE A REALLY GOOD REASON TO NEED MORE (HINT: YOU PROBABLY DON'T)

Blade of the Ruined King High damage, slow acts as an anti-kite and mini-nuke. Good for when the enemy carry is slippery and for winning duels.

Wit's End More magic damage on hit in conjuction with devourer. Diversifys your damage output and adds tankiness.

Statikk Shiv Decent damage on the proc and gives some mobility as well as some crit and attack speed. Nice little bonuses on an item that supplement the rest of the kit.

My god I'm dying!:

Calm down my bro or broette. Just, get something tanky, take a step back, and then charge into their face again. You got this.

Killed by magic damage?
Then you my friend need a Banshee's Veil. Nothing says getshrekt like two spellshields. 2spooky.

Killed by physical damage?
Sunfire Aegis. Beautiful item. Everything slowed so they cant escape, and they are forced to either tank the damage or leave the AoE.

Killed by a mix, or just need a good all around item to get you firmly into tankville?
Turbo Chemtank. CHOO CHOO HERE COMES THE NOCTURNE. So let me explain. Firstly, you activate this item while near your team. Secondly you ult into them immediately afterwards. Thirdly you use your movement speed that is still there to stick like glue to their carry while fearing them. Finally the speed ends and they all get slowed. This item is so almost core I would recommend picking it up nearly EVERY GAME.

Bonus Utility:

Just some items that work nicely that help to add some additional utility into the build, while being concordant to the theme.

Spirit Visage: MR, HP, CDR, some nice regen and some extra healing for good measure. Overall strong item.

Frozen Mallet: Sticking power. If you haven't noticed, nocturne is about as sticky as cheap sellotape and so helping that is important. (Yes he has dusk and fear, but after that people can get away if they are smart.)

Locket of the Iron Solari: Nice if your support isn't rocking the locket. Good active, strong overall stats, nice to get in a situation where overall team survivability is an issue.

Warmog's Armor: Post 420 (the blaze it patch) this item is a little stronger. I personally don't like the item, but in longer fights or more fragmented fights this item can help a lot. Situational.

Guardian Angel: Why not? Tanky, utility, and a big middle finger to their team when they have to kill you again. :^).

Many initiators focus on instant CC, think something like Malphite. He goes into the fight and then messes your day up. He can then survive and use limited cc and damage to continue the fight. You see a guy like malphite, and you know your face is about to get squished like a ripe tomato. It's par for the course. It just is. He is oppressive in his mechanics, it is the certainty of being dived on.

Nocturne however has a subtly different scenario. He ults, and panic ensues. The enemy team suddenly lacks vision, the single most valuable asset to a team. The raw power hidden in this is ASTRONOMICAL. Then a tanky guy jumps on you, you panic, you blow your cc as he hits your carry, but he just ignores it with a spell shield and uses it to just hit your carry more. People start to panic, as your team jumps in and uses this chaos to mop up. The panic, that hidden psychological element to Nocturne's initiation is what makes him so strong. People know the malphite will jump. People know the jarvan is going to be helping. But when that nocturne ult goes down, their team doesn't know, and doesn't see. That is why Nocturne.

That could be all, but the extra element to all this is Nocturne can deal a surprisingly high amount of damage with just two damage items, one of them being his devourer. This makes him a high threat that the enemy team just can't ignore.

Start red. Smite to finish, not to start (obvious, but needs mentioning.) Smiting red will give you a 20% max hp heal.

Then you have a choice:
Option 1: Clear Raptors (wraiths) or Wolves while waiting for smite cooldown to kill blue.
Option 2: Wait at blue buff for smite and then kill blue.
Option 1 makes following on into a gank a little harder, and option 2 prevents blue from being counterjungled and gives you more hp for a follow-up gank.

After red and blue and a likely gank, go back and get the skirmisher's sabre to help your clear speeds, and get more health potions. ONLY ONCE YOU NOTICE YOU DON'T NEED THE HEALTH POTIONS TO STAY NEAR MAX HP DO YOU NOT HAVE HEALTH POTIONS.

At this point, try to find a balance between clearing camps and ganking. Remember to communicate your ganks, and if your gank path is warded, try a different path, or try using the raptor (wraith camp) smite bonus to deward, if you did not take the red trinket.

COMMUNICATION.

No seriously, talk to your team. Tell them you will be building a tanky Nocturne and will act as an initiator. Let them know that they can follow up. Give them confidence in you. When ganking, talk to them. Tell them when you plan to gank for their lane. 'I'm just getting red, I'll be ganking bot through river in 20 seconds from now' is a lot more useful than jumping into bot lane with no forewarning. Even if you get the kill your teammates may have needed to flash to catch up, you never know. COMMUNICATE. Be the initiator you want in the world. Yes I just semi-quoted Gandhi, but that guy was challenger tier you know, took down the caste system in the LCS.

Basically, be a nice guy who communicates and people will think you are a nice guy and listen to you. It works surprisingly well.

Assuming you have Turbo Chemtank, use it while with your team, after communicating your incoming engagement. Note that this item active acts as a nice IM GOING IN NOW signal, not that your ultimate isn't. If you don't have RG, just skip that bit (leave in the communication, even if its just a ping) Follow up with an ultimate.

Once you have ulted, dive onto their carry. When in the middle of the team, use Randuin's Omen active. Once close to the carry use your Q on them, followed by fear on them, unless they have some form of distance closer, then try to spook hem into using their distance closer, then use your fear so it is harder to break it. Keep in mind you can (and should!) smite a champion to reduce the damage they deal to them and deal bonus true damage to them on hit. You don't necessarily need both of these effects (that is, you don't need to utilise both the damage reduction and bonus true damage) : fearing their carry leaves you free to stop the burst mage from hitting so hard (by smiting them), for instance. Remember you have a spell shield: use it to prevent counter initiations and to waste one (or two, with banshees) of their spells.

By this time your team should be in the fight. Now your role shifts depending on the circumstance:

They have scattered and are running:

Chase. Nocturne is great at this.

They are in somewhat of a position to fight back, though naturally disadvantaged of a few spells and likely behind hp wise:

Focus on removing (note: you don't need to greedily chase them, dead dps and too low hp to be in the fight dps are both ZERO) their carries first and foremost, at the very least their ADC, which should be out of the fight. Then choose to either protect your carry or deal more damage, which is the right choice depends on how much potential they have to kill your carry, and how much cc your support has to peel for the carry on their own.

As always, use your best judgement for the situation.

BUT WHAT IF THEY JUMPED ON US?

Deep breaths my child, deep breaths.

Firstly, don't stupidly waste CC on their initiator unless you MUST peel for your carry and you can kill them before significant help from their team arrives. (note this happening to you is worst case scenario, and is why communication is key.)

Your primary goal is to act as a tank, making their efforts to take out your delicious squishy back line futile. Your main focus is to stabilise the fight, neutralising their momentum and then pushing back with your own.

If you can manage this, and get the situation stabilised so that both teams are somewhat even, you can then do what the magnificent nocturne does and dive onto their back line. As always, no generalisation will fit every situation and so your best judgement is always important.

In summary, all of the above. Build tanky items with supplemental CC or interesting effects, with Randuin's and Righteous Glory being prime examples of such, act as a hard frontline initiator for your teammates to piggyback off of. I would say more, but I would only be repeating myself.

Good luck, have fun, and thank you for reading.