Nocturne is Assassin Melee Champion, with four excellent abilities, which make Nocturne great killer of opponents one on one. You can make full combo from abilities. You can first use Paranoia, then quickly fear them with Unspeakable Horror, while using activating Shroud of Darkness (maybe lucky and start attacking with more speed) and finaly, following them with Duskbringer. I prefer Nocturne playing in lane, because you're a farming machine and getting minions is the most simple way. Best to play him in lane with partner and double killing is not a problem, just use Unspeakable Horror and we are done.

1. I buy Cloth Armor, so you don't get hurt by minions well. Then buy Madred's Razor, exellent item for critical striking minions. End it with Wriggle's Lantern.

2. Buy boots for attack speed. Perfect item for starting killing minions faster and gaining more gold for other items.

3. Again, minions are your primary mission, so Dagger, Zeal, Dagger and Phantom Dancer.

4. Buy B.F. Sword and end it with The Black Cleaver. It will give you extra attack speed and attack damage.

5. Buy B.F. Sword and end it with The Bloodthirster. New attack damage and very important life steal.

6. Buy Ruby Crystal, Catalyst the Protector and finish it up with Banshee's Veil. This item gives you extra health and mana, so you can stay longer on lane or in fight. Give some extra magic resistance and blocks one negative speel every 45 seconds.

7. Sell Wriggle's Lantern and buy Phange and then Frozen Mallet. It will give you extra health and attack damage, not to mention that slow enemy movement speed by 40%.

Duskbringer
(Active): 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.

* Cooldown: 10 seconds.
* Range: 1,200.

Cost: 60 / 65 / 70 / 75 / 80 mana.
Physical Damage: 60 / 110 / 160 / 210 / 260 (+0.75 per bonus attack damage).
Movement Speed: 15 / 20 / 25 / 30 / 35 %.
Attack Damage: 15 / 25 / 35 / 45 / 55 .

Try to use this ability when killing minions in lane. Try to hit enemys as well, they will move back for some seconds and you have time to kill some minions with you Innate. You can use it when following or running from enemy.
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Shroud of Darkness
(Passive): Nocturne empowers his blades, passively gaining attack speed. Attack Speed: 20 / 25 / 30 / 35 / 40 %.
(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.

* Cost: 50 mana.
* Cooldown: 20 / 18 / 16 / 14 / 12 seconds.

This ability is awesome. Have finger always prepared for this skill cause, you can stop anything with this. Like vladimir's pool, you can escape from magic (karthus ultimate, ash ultimate, tryndameres slow, even stuns). In adition you gain attack speed. I use it, when going into team fight, cause you never know, when they will try to stun you, so good defence is the best offence.
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Unspeakable Horror
(Active): Nocturne plants a nightmare into his target's mind, dealing magic damage over 2 seconds. If Nocturne stays within range of the target for the full duration, the target becomes feared.

* Range: 475.
* Leash range: 900.

Cost: 60 / 65 / 70 / 75 / 80 mana.
Cooldown: 15 / 14 / 13 / 12 / 11 seconds.
Magic Damage: 50 / 100 / 150 / 200 / 250 (+1.0 per ability power).
Fear Duration: 1 / 1.25 / 1.5 / 1.75 / 2 seconds.

Use it only in fights against enemy champion and when attacking dragon, baron, lizard or golem. You can use it as well when running from opponent. You can get some time when fearing him.
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Paranoia
(Active): Nocturne reduces the sight radius of all enemy champions and removes their ally vision for 4 seconds, this includes champions, minions, wards and towers.

While Paranoia is active, Nocturne can launch himself at an enemy champion, dealing physical damage.

* Cost: 100 mana.
* Sight modifier: -300 (estimate).

Cooldown: 160 / 130 / 100 seconds.
Physical Damage: 150 / 300 / 450 (+1.2 per bonus attack damage).
Range: 2,000 / 2,750 / 3,500.

I use this ability when enemy is too far away and i want to get there as quickly as i can. I use it for diving under turret too.
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Going one on one
Use abilities in this way:

Okay people! This is it! I hope I helped someone. My first build and I come from Slovenia, so my English is poor. : D