Nocturne is a very fun champion to play and, in my opinion, the funniest assassin. His paranoia is pretty scary to the enemy team. Hes also a great team fighter and initiator but if he's squishy, he'll die instantly even with his shield. This build will allow you to kill and build up some defense so that you can assassinate without dying.

Going up the Offense tree will help you kill earlier. As a melee character, Nocturne has his runes for survivability and should focus on dealing dmg.

- The blue runes can vary. I chose to use CD reduction simply because you can't really go wrong with them. You can also get MP-5 runes if you want to harass more.

Greater Seal of Vitality - The HP yellow and quints greatly increase your survivability to gank/asassinate early game. By level 6 the per level yellow HP runes will give more HP than flat ones.

Greater Mark of Desolation - The red armor pen also helps kill early game with the Dusk-bringer -> auto-attack combo (not really a combo...)

This is it. The maker and breaker of every character. Nocturnes, or carries in general, usually nuke like crazy but also die like crazy. If you can get past the bulk of the team to the back where the carries are, eliminating them before they eliminate you will usually win the team fight. With nocturne it's surprising easy with Paranoia.

- Start with a Long Sword and Health pot, the Long Sword and your passive will allow you to lane longer and the health pot will keep your health up if there harassing you too much. The long Sword will build into a Brutalizer later, so it'll save you money

- Immediately build your mercury treads, this item is one of the most useful for speed, debuff time reduction and MAGIC RESIST

- Zeal should be built from your dagger to get the most out of your early game

- BF Sword is next so that you can deal a lot more dmg, this will compliment your paranoia so that you can get more successful ganks

- The Phage will increase your health and slow opponents

- Infinity Edge should be bought before the Phantom Dancer because it increases your DMG and early on for harasses (Dusk-bringer).

- Phantom Dancer for Crit and the movement speed is nice

- Depending on your deaths, you can finish the Frozen Mallet before the Phantom Dancer for increased survivability. Now no one can escape you

- Atma's Impaler is a very underused item. With your HP so high now, this items passive will give u an extra 60 or so atk and, above all, CRIT CHANCE and ARMOR

- This items is great because of crit chance, atk, and amazing active...not to mention armor pen

Dusk-bringer - Your greatest harass, so max it first. Also scales nicely with attack damage

(Active): Nocturne throws a shadow blade which deals damage, leaves a Dusk Trail, and causes champions hit to also leave a Dusk Trail. While on the trail, Nocturne ignores unit collision and has increased Movement Speed and Attack Damage.
Cooldown: 10 seconds
Cost: 60 / 65 / 70 / 75 / 80 mana.
Physical Damage: 60 / 110 / 160 / 210 / 260 (+0.75 per bonus attack damage).
Movement Speed Bonus: 15 / 20 / 25 / 30 / 35 %.
Attack Damage Bonus: 20 / 30 / 40 / 50 / 60 .

Shroud of Darkness - Its shield is useful and the passive is great at its first level. Every other level doesn't improve his shield and only increases the passive by 5% after the initial 20%. So only one point is needed. Max this LAST.

(Passive): Nocturne empowers his blades, passively gaining attack speed. (5% per Level)
(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.
Passive Attack Speed Bonus: 20 / 25 / 30 / 35 / 40 %.

Unspeakable Horror - Max second, The fear helps ganks a lot and can save your team mates.

(Active): 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.
Range: 500.
Cost: 60 / 65 / 70 / 75 / 80 mana.
Cooldown: 15 / 14 / 13 / 12 / 11 seconds.
Total Magic Damage: 50 / 100 / 150 / 200 / 250 (+1.0 per ability power).
Fear Duration: 1 / 1.25 / 1.5 / 1.75 / 2 second(s).

Paranoia - Level up whenever possible because this is your frosting on the cake.

(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. He can then launch himself at a enemy champion with a large range. Deals physical damage on arrival.
Cost: 100 mana.
Cooldown: 160 / 130 / 100 seconds.
Physical Damage: 150 / 300 / 450 (+1.2 per bonus attack damage).

Flash - One of the best skills on every carry, this greatly improve your getting in and out and escaping (that's what she said).

Exhaust - Helps dusk-bringer hit and allows you to hit the enemy more, not to mention you can use it to escape.

Ignite - The extra damage is pretty useful but exhaust will allow you to land more hits and your physical attacks will always end up greater than ignite due to dusk-bringer

Ghost - increase in survivability isn't bad, can also improve ganks.

Heal - NO

Revive- Initiate a team fight, die, revive, rush back to the team fight with Paranoia and clean up...I hope you realize I'm joking by now...

Lane Partner - The lane partner should either be a ranged carry or tank. Lane dominance is important and easy with Nocturne. The lane partner should also be able to solo lane for small periods of time while you gank. Nocturne doesn't need to solo lane nor shouldn't in case you have a jungler.

Dusk-bringer is a great harass and u can also sweep in and auto-attack after. You passive will heal off the damage and the Health pot is there for when you deem fit.

When you get Paranoia, you can start ganking enemies with low health. When you get your BF sword you can start ganking lanes with the help of your team in every lane. When ganking, blue and red buff would help greatly, so get them if you don't have a jungler.

If your ganking and the set-up is taking a loooooong time, just jungle. Blue buff and red buff are both extremely useful on Nocturne so continue to get them.

Team Fights
Nocturne doesn't have to be AT the fight with Paranoia, He can effectively clean a lane or jungle until the teams stop dancing around and fight, its best because during paranoia the sight radius is reduces and you can teleport to a carry after your tanks absorbs the initial damage of the fight. Be quick, don't let your carries die, use paranoia ASAP. Being away from the initiation will make the team think your not there and fight as a 4 vs 5.
Don't forget your Shroud of Darkness, in team fights it'll save your life. Use it when initiating on a carry. They usually panic and pop a strong move/stun that your shield will absorb.