Nocturne is a malee DPS, assassin and ganker. He can catch enemy when really far away from him. He can be good jungler and awesome liner.

It's my first guide, so i have to say: "Hello". (; My english isn't very good but I think you won't have problems with understanding me. I do my best anyway.

Pros

Awesome Damage
Great Ganker
Good Liner and Jungler (passive)

Cons

Squishy
Sometimes mana problems, especially without blue buff

Umbra Blades (passive)
Every 10 seconds, Nocturne's next attack strikes surrounding enemies for 120% of his physical damage and heals himself for 15 / 20 / 25 per target hit. Nocturne's basic attacks reduce this cooldown by 1 second.

Just wonderful passive. Makes u great jungler and liner as well. Really good to harras (Duskbringer+Umba Blades). Makes u big possibility to line long, without recall.

Duskbringer
Nocturne throws a shadow blade which deals damage, leaves a Dusk Trail, and causes champions who are hit to also leave a Dusk Trail for 5 seconds. While on the trail, Nocturne ignores unit collision and has increased Movement Speed and Attack Damage.

And thats why enemy cannot escape you and you deal huge damage. If you exhoust your target on Dusk Trail he have to die. Or flash. (; Can be used for run too.

Shroud of Darkness
(Passive): Nocturne empowers his blades, passively gaining attack speed.
(Active): Nocturne creates a magical barrier for 2 seconds which blocks the next enemy spell. If an ability is blocked by the shield, his passive attack speed is doubled for 5 seconds.

Just Double Banshee's Veil in skill. It's hard to use, but if use well, it's can save you really often. And passive Attack Speed - wonderfull thing for fighter.

Unspeakable Horror
Nocturne plants a nightmare into his target's mind, dealing magic damage over 2 seconds and fearing the target if they do not get out of 600 range of him for the full duration.

Your Crowd Control. Really important when playing against channeling skills (Fiddlestics' Drain, Nunu's Absolute Zero, Katarina's Death Lotus). Dealing some damage and making enemy fear. Give u more possibilty to make effective gank and can save you when chasing by fighter.

Paranoia (ultimate)
Nocturne reduces the sight radius of all enemy champions and removes their ally vision for 4 seconds, this includes champions, minions, wards and towers. He can then launch himself at a enemy champion with a large range. Deals physical damage on arrival.

Nightmare's ultimate. Every time you use it, enemies run. It's not important are they stronger than you, or not. Just run to tower, becouse they can see nothing and don't know what to do. After that u can teleport to enemy champion, deal big damage and kill of course. It's can be used for escape, but it isn't work as that often.

Good Summoner's Spells

Ghost
Great spell for Nocturne, give you possibility to switch lines fast, gank often and chase running enemy. It can save your life too.
Exhoust
Great spell too. Slowing enemy if he want escape and damp if he want fight.
Smite
Must have for jungler. Especially good against strong minions like Annie's Sommon Tibers, Malzahar's Summon Voidling or just Super Minions.
Ignite
More damage - more kills. Especially against High Healing Champions, like Dr Mundo or Warwick.
Flash
Chasing enemies and running.
Clairvoyance
Great Synergy with your Ulitmate. You can teleport to target only if you see him. Good to check bush, Dragon or Baron.

Bad Summoner's Spells

Heal
Oder spells give you more chance to survive than Heal.
Revive
Too long cooldown, too small effect.
Teleport
Your ultimate making you dont need teleport.
Cleanse
Your Shroud of Darkness is the same and more than Cleanse.
Fortify
Fortify works really good if you have Masteries in it. You, as Nocturne, have not. Tank should get it.
Clarity
If you need more mana you doing something wrong. Maybe try get Golem Buff or stop wasting skills on creeps? ;)
Rally
You are not in teamfight from beginning. You jump into in middle and stay there few seconds. You can't get all bonuses for team it gives for team when playing Assassin. Support/Fighter should get it.

First, important thing: go line with champion, who can line 1v2. You shouldn't mid, it makes it line often empty.

Harrasment: Use your Duskbringer when have passive ready. Go fast on your Dusk Trail, hit him and, if lucky, another champions and creeps near. Back fast, before Dusk Trail diseapear. You are really strong early so don't afraid fight. Often can score First Blood and die, but your line partner should kill second enemy too. So it's 2:1 for your team.

Don't waste mana often. You need it to gank with your combo.

It's in my opinion worse way, but works too. It's especailly good if u have got runes for jungle and don't lose many HP in jungle. Build change is switchDoran's Blade forWriggle's Latern. It gives more Damage, more Lifesteal and Armor instead Health. And it's more expensive of course.

Jungle Map

Red Way
Way for everybody but don't giving you red buff.

1. Wrathes
Smite the strongest one and Duskbringer oders. Attack them on Dusk Trail. After kill use potion and go to wolves. You will kill them on the same potion.
2. Wolves
Duskbringer and kill wolves. I prefer start from weak wolves. You have 2 level. Take Shroud of Darkness.
3. Blue Golem
Duskbringer Golem and two lizards (try how do that on custom) and attacking in place, where you can hit all these minions from passive. Use potion, or two, if needed. When Duskbringer recharge use it again.
4. Small Golems
Duskbringer, potion and beat. (;

Then wrathes should respawn. Kill them the same like first time. Then wolves and recall.

Blue Way
Easy and with red buff but runes equired.

The same like in Red Way u using Duskbringer and attack. With Lizard u can use Unspeakable Horror for easiest kill. Smite use on Golem and Lizard. Potion with all way stations.

It's your destiny when playing Nocturne. Gank, gank, bit jungle and gank again. Your combo isn't hard and very effecitve.

Really awesome Nocturne's point is ganking right from jungle with his Ultimate. You can beat Golem and just in second gank mid. When you teleport to enemy use Duskbringer and Unspeakable Horror. If your target has got Crowd Control (like Ryze's Rune Prison, Fiddlestics' Fear, Ashe's slow or stun) use at once Shroud of Darkness. It will give you (if blocked spell) more Attack Speed and more chance to kill.

If you attacking without ult start with Duskbringer from bush and fast attack. If enemy has Crowd Control - use Shroud of Darkness when emanate to target. As fast as is possible use Unspeakable Horror and attack.

If enemy useGhost or start fight with you (especially fighters)exhoust him.

Berserker's Greaves
I think this boots are only good for Nocturne. Even playing against Hard Magic or Hard Physical Team they give you best survival rate (Lifesteal and Passive).
Stark's Fervor
Sometimes I end game with over 5000g and only this item. It's just perfect for Nocturne and kills everything in Early-Middle Game. It gives you Lifesteal (basic survival for Nocturne), Attack Speed (more heal and damage from passive), Aura to effective gank and Health Regen to heal more beetwen ganks.
Youmuu's Ghostblade
Another great item for Malee Assassin. It gives you Cooldown Reduction (important for Ulitmate),Attack Damage, Armor Penetration, Critical Chance (important a bit later) and awesome Attack Speed when active.
Phantom Dancer
More Attack Speed, Critical Strike (starting working now), and Movement Speed.
Infinity Edge
It gives u a lot of Damage, and Critical Strike Damage. Now all your attacks are critical and you deal 250% Damage.
The Bloodthrister
If game isn't end yet you will need more Lifesteal to survive. This item gives u realy big Lifesteal and Damage boost.

I think you won't need other items. Why? Becouse best deffense is offense. Whenever you been focused in teamfight you will die, indenpendently items you got.

Of course if you playing against five Hard Physical Malee Fighters you should getThornmail. The same if you playing against five Nukers - you should getForce of Nature. But... these teams are not exist. You will never play against it, so you shouldnt change your Core Build.

If you playing against High Armored enemies you shouldnt buyLast Whisper too. You have enough Armor Penetration fromYoumuu's Ghostblade and runes.

One reasonable and sensible Situational Item isGuardian Angel. If you getting down by Crowd Control in teamfight you will back and steal life again. You can take Guardian Angel insteed The Bloothrister if enemy have got a lot Crowd Control.