Now the first thing you're probaly wondering is, "How is a crit chance build at all good for Nocturne?" Well throughout this build I will explain how to use this build and why to use it so that yo can "Slice up" your opponents.

When I first got Nocturne, I went through atleast five different builds all centered around Attack Speed, Life Steal, you know, the obvious for a bladed DPS. They all seemed alright for a champion such as Nocturne but nothing stood out. He just didn't seem to be able to keep up with someone such as Tryndamere or Kog'Maw. It hit me that thanks to his Shroud of Darkness passive that he didn't need any Attack Speed items till late game. Also that his base damage + items was decent enough early game to go against champions. So... Since he's already decent early game, why not give him a build that doesn't boost his early game capabilities, but instead give him a build that makes him ungodly late game. Critical Chance.

Exhaust- You will need this spell throughout the game till you get decent crit chance if you're going up against DPS carries. This will completeley even it out.

Clarity- Nocturne is one of the most mana hungry champs I have come across. This is needed throughout the entire game since he won't be getting any mana items till Banshee's Veil. Now why not ghost? Because I have found that all I need is Exaust in order to catch enemy champions, even without it. I have very rarely have a champion out run me unless there was a turret near by.

Pros-

Since Nocturne is pretty strong early game, this build keeps him right up with other builds and gives him insane strength late game.

This build prioritizes what Nocturne needs first.
Life Steal--->Damage&Cooldown--->Damage--->Attack Speed&100%Crit Chance

The last item is substitutional by a variety of items depending on what you need.
Magic Resist/Armor/Health/Ect.

Cons-

The main conflict on any build with Nocturne is that he is a 1v1 champion. If you rush in to teamfights and you're outnumbered by 2, NOCTURNE IS NOT A CARRY. He is meant to effectively take out enemy champions 1 by 1 or chase down the stragglers that get away. Right now, Nocturne is seen as a major threat so in most teamfights, you'll be targeted first.

You need to learn how to use Shroud of Darkness, without it, if you're going up against a champion with a stun, you're bound to get destroyed.

Duskbringer- If you are chasing and you fail to land this, say good bye kill. It is quite powerful, and by the time you get Ghostblade, it's equal to one of your melee hits.

Shroud of Darkness- You need to learn how to use this. Get over it. If you're going against more the one champion and you're alone, and you fail at the timing you'll die. However if you can watch for particular movements, like a Sion or Teemo charging at you. They're probaly gonna use a ranged ability. It's not hard once you notice patterns. This is a lifesaver. It will block any ability, even ultimates. So if you're being hunted by a Warwick and he's got oyu just about in range for his Infinite Duress, just turn around activate Shroud of Darkness and let his ulti fail. The passive of this ability also increases his attack speed as you level it up.

Unspeakable Horror- Definetely a must in a 1v1 or running away. The second you charge in or you need to escape, latch on to the enemy with it. Fear is Nocturne's friend.

Paranoia- One of the most beautiful ultimates you will ever use. Save it for guarenteed kills. Try hiding in bushed or right behind walls to use it. The best gank ultimate you will ever find as well. Personally I think it's better than invisibility because you get where you need to fast.

Nocturne is a very flexible champion pertaining as to who he can lane with. I would not recomend laning with a tank though(unless it's someone with a stun like Sion). Try to stick with someone who has range or a champion who can take hits like Tryndamere.

Grab two health potions because they're about all you will need till you get Executioner. Once you arrive at your lane, do not head to the grass unless you know no one is there. Nocturne isn't really capable of much till he gets to level 3 and has all three spells. Just stay behind minions, get hits in on them every once in a while to heal yourself when needed. Keep in mind it's very easy to push Nocturne back if his abilities on cool down so don't harrass to much with him. You should be constantly watching your opponents movements and practically have your finger on the hotkey for Shroud of Darkness constantly.

So it's been about 10 minutes, you should have Beserker and Executioner by now providing you've gotten a kill or two. You should NOT use your ulti unless you know it's a guarenteed kill. It's not worth the cooldown. At this point, you should be able to push back the enemies with just your melee attacks. It is also around this time that you should gank mid with Nocturne's ulti.

It's 20-35 minutes. Now you can effectively turret dive on a single champ without dying providing they don't have full health and aren't a tank. Depending on how many kills you have, you may very well have Infinity Edge by now which means you're dealing 600+ per critical. You're running around taking out the champion who are by themselves while your teamates are handling the groups or you're saving teamates from chasers. Golem buff is necessary to have now so you knock off a good 30 seconds from your ulti. You should be able to farm minions so fast you don't even need to rely on champion kills to get gold now. Aim for Zeal next to add on a good 8% movement speed and make yourself even faster. Now you can catch anything that you hit with your Duskbringer making Ghost pretty much obsolete offensive scenarios.

[Reminder]
You should have Youmuu's Ghostblade by now, remember to use its' UNIQUE active!

40-55 minutes. Now you have Phantom Dancers hopefully and now you should easily be able to handle any champion 1v1 or in some cases even 1v2. Your ulti should be down to a cooldown of about 70 seconds which makes you a deadly force to any lone champion. The second the enemy hears "Darkness..." as their minimap goes away and the screen darkens.... They know someone somewhere is getting weeded out.