This is me enjoying a new champ and wanting to share that. It's my first guide, and is based, just a bit, on guesswork. Take all that into account before you troll me. Now to my purpose:
Nocturne is an inescapable terror. With his lovely cruise missile ult and beautiful speed he is not someone you outrun. This build works with his ability to chase and harm folks.


I have been working on it a little, adding and expanding.

Not hard, although part of my presentation there skews the stats in the final totals box.
You will want armor pen, so i took marks and quints in it. I take health seals for just a drop of survivability, although dodge might be fun too if you're expecting a lot of physical dps (that's why i put em both in runes section). I take CDR because it's easier to dole out the harshness when your abilities are ready to go, especially with Trin Force.

Straightforward shredder. 22-0-8. All your crit chance, some exhaust plus, more CDR, more AS, more ArPen, and then it's all hurt with a bit of better ghost, quicker heals and levels. Tasty.

I only have five posted, because so far those are the only five that i've built more than once or twice. Leave the last for situational stuff.
The order is pretty much up to you, although i start with a dagger. More attack speed means I deal more damage with my passive (because it procs more) and can heal more often. I take this, rather than a doran's item, because it builds right into my next items, rather than being a throwaway later. From there I'll try to finish phantom dancer, an move on into my boots. Why not berserkers? Because after the dancer, trin force, bloodrazor, and the passive on shroud of darkness, you really don't need it. I build swiftness so that I get more use from speed buffs of Dancer and Trin force. Between the on-trail speed of Duskbringer, your passive move speed, and the insane rocketing with ghost, you can get anywhere in a matter of seconds, and your enemies can only get to the place you kill them. After that it's just a matter of riding your Trin procs and wicked AS/AD/lifesteal combo to the salad bar.

I won't tell you level by level how to do it, because you're all big kids and can figure that out yourself. I start with Duskbringer, and try to max it first. After that I work on Shroud, then Horror. My reasoning is simple. The first two help you chase chase chase and shred shred shred. Horror is fun for keeping someone locked in for a minute once you catch them, but it's not as useful (in my opinion) as the others.
*editor's note: I didn't want to tell you how to do it, but Moba made me. Pretty much said if i wanted people to see it much, i'd need to. Sorry if I can't remember right off where the ults get improved, try to fix that later.

The short version is that you have exhaust to further increase the gap in your speed, as well as help in fights with other melee champs, while having ghost to move at extreme speed. Throwing exhaust on a pursuer and then ghosting away is a pretty solid way to outrun a gank.
That was also the long version.

Fun as this build is to play, it does have some drawbacks. The biggest one is that it leaves out survivability in favor of lethality, although the sixth item slot is still open in case you want to throw some into the mix (fun note: Force of Nature will give you great passive regen, MRes, and even MORE SPEED). Also, early game can be a bit of a hassle, as you're pretty much the squishiest little guy outside of yordles. My answer to that has always been to simply play better and don't get caught overextending. Caution, caution, catch em nappin, kill. Four steps to early game pwn.

I refer to the four steps to pwn that i mentioned above. This build is very lethal, if used correctly, and absolutely useless if not played intelligently. With it you will be able to kill squishies incredibly fast, tougher carries in a couple/few seconds, and tanks with only a bit of work. You will not be able to do this in the middle of their team, unless a fight is already going, and even then it's a bad bet. Kill the stragglers, the overextenders, and use your ult to completely disrupt the enemy cohesion in teamfights. It's hard to focus one person down when only one or two of your team can see him, right? Then the light comes back and your Ashe is layin cold at the feet of a grinning shadow, which jets away the instant you try and hit him.

In this section, will address the question of the sixth item. There are several good choices floating around, depending on what you want to do. I'll divide them into Survivability, Attack (speed and damage), and Movement, being that the first is kinda left out in this guide, and the others because they are what this build is all about.

SURVIVABILITY:
Shurelya's Reverie: +330 Health +30 Health Regen per 5 sec +15 Mana Regen per 5 sec UNIQUE Passive: Reduces ability cooldowns by 15% UNIQUE Active: Nearby champions gain 40% movement speed for 3 seconds (60 second cooldown) More health, more regen of mana and HP, lower CD, and an extra ghost function? Seems pretty nice.

Randuin's Omen ; +300 Health +80 Armor +25 Health Regen per 5 sec. UNIQUE Passive: 5% Cooldown Reduction and 20% chance on being hit to slow the attacker's movement and attack speeds by 35% for 3 seconds. UNIQUE Active: Slows movement speeds and attack speeds of surrounding units by 35% for 2 seconds + 0.5 seconds for each 100 Armor and Magic Resist. 60 second cooldown. Another interesting item that adds beef and Hp5, and trading Mp5 for armor. It also carries CDR and a great passive slow, while also packing an even better AOE slow.

Warmog's Armor: +920 Health +30 Health Regen per 5 sec Passive: Permanently gain 4.5 Health and .15 Health Regen per 5 sec per minion kill. Champion kills and assists grant 45 Health and 1.5 Health Regen per 5 sec. Bonuses cap at +450 Health, and +15 Health Regen per 5. In a word: BEEEEEEEEEEEEEEEFFFFF

Aegis of the Legion: +270 Health +18 Armor +24 Magic Resist UNIQUE Aura: +12 Armor / +15 Magic Resistance / +8 Damage to nearby allied units. This gives a little bit of all your defenses, and gives it to your teamies too. It's pretty cheap, too.

Force of Nature: +76 Magic Resist +40 Health Regen per 5 sec +8% Movement Speed UNIQUE Passive: Restores 0.35% of your champion's health every second. This takes casters down a notch, while helping your regen and move speed. Useful.

Banshee's Veil: +375 Health +375 Mana +50 Magic Resist UNIQUE Passive: Blocks one negative spell every 45 seconds. Pretty firm Mres, and another spell shield. It'll do.

Spirit Visage: +30 Magic Resistance +250 Health UNIQUE Passive: Reduces ability cooldowns by 10% and increases your healing and regeneration effects on yourself by 15%. It helps you heal, and that's always nice, especially with some CDR.

ATTACK:
Infinity Edge: +75 Attack Damage +20% Critical Strike UNIQUE Passive: Critical hits now deal 250% damage instead of 200%. With all the crit chance you have, this baby will make your damage output positively cataclysmic.

Black Cleaver: +55 Attack Damage +30% Attack Speed UNIQUE Passive: Physical attacks reduce your target's Armor by 15 for 5 seconds (maximum 3 stacks). Even more AS, and armor reduction. Yes please.

Yoummuu's Ghostblade: +30 Attack Damage +15% Critical Chance UNIQUE Passive: +20 Armor Penetration 15% Cooldown Reduction UNIQUE Active: You gain +20% movespeed and +50% attack speed for 4 seconds. Attacking enemy units with melee attacks increases the duration by 2 to a maximum of 8 seconds. 60 second cooldown. AD, AS, ArPen, CDR, active movespeed buff. Solid contender.

Stark's Fervor: +20% Attack Speed UNIQUE Aura: Gives nearby allied Champions 20% Lifesteal, 20% Attack Speed, and 30 Health per 5 seconds regenerated. Reduces the Armor of nearby enemy champions by 20. Another item that adds AS, along with even more life steal and armor reduction. Oh, and your teamies get those last bits too.

Tiamat: +50 Attack Damage +15 Health Regen per 5 sec +5 Mana Regen per 5 sec Passive: Your attacks splash, dealing 50% area of effect damage around the target (35% for ranged attacks). This one is fun because of the way you can hit everyone at once, and you gain regen.

Hexdrinker: +35 Attack Damage +30 Magic Resistance UNIQUE Passive: If you would take Magic Damage which would leave you at less than 30% of your Maximum Health, you first gain a shield which absorbs 300 Magic Damage for 4 seconds. 60 second cooldown. This gives some damage, and a bit of defense against those pesky mages.

MOVE SPEED:
Frankly, I've exhausted all of the movement speed options that are beneficial in the items above. I guess this section wasn't really necessary. *shrug* I guess you could go for boots of mobility instead of swiftness, but you trade some of your speed in combat for speed outside of it, and I don't see it being worth it.

In summary: You will move faster than anyone and shred folks. That's fun, so long as you're smart with it. Now, bring on the trolls.