This guide offers Nocturne users damage, speed, soft single-target crowd-control, and survivability for maximum effectiveness. Be advised that build items should be changed depending on what you're facing. For example, if you're facing two or more tanky champions (ex: Rammus and Shen), you'll definitely want to rush Madred's Bloodrazor first thing instead of Frozen Mallet or Black Cleaver. This build offers up a guide to what the optimal Nocture build will look like. Enjoy!

There're two build variants (so far) that guide Nocturne users to victory. One is the glass-cannon, which offers high damage and survivability, but still you're a glass cannon and have to play smart. The other build is a more beefy variant that employs the Frozen Mallet for amazing health and soft cc. The Mallet ensures that your fear will hit, and it also aids in keeping targets in place so that your allies can move in for the kill. Either way, both are stunningly effective for Nocturne.

Mark of Desolation - More Damage. They offer the best damage out of all the physical attack runes by far.
Seal of Clarity - Nocturne is mana-hungry, and needs it to do work. His skills scale in cost, so Clarity runes which scale evenly will aid you in staying full. You still cannot spam your skills until you've acquired the blue golem buff, though.
Cerelity - Mainly for reducing the recharge on his ultimate skill (R), but it also helps his other skills. Rushing the Brutalizer and blue golem buff will aid Nocturne in having a decent cool-down on his ult while he levels so that by the time he reaches 18 he'll have the absolute best recharge possible without compromising his survivability (i.e.: Merc Treads > Lucidity Boots).
Desolation Quints - Same as above, you'll want to be able to dish out as much damage as possible. All other runes pale in comparison to these babies.

Standard AD Skill Mastery Set up. You want all the critical strike, attack speed, and bonus armor pen and what not to aid you in early game, and even mid to late game. You can choose to invest in Ghost and add a point there. If you go with flash, don't worry about trying to get a point in the utility section because depriving your offense for it just isn't worth the 15 seconds. Simply put that point in reduced death time so that you can come back faster. With the patch that brought us Nocturne, Riot made late-game death timers longer, and so putting even 1 point in there helps!

Long Sword (or Ruby Crystal, depending on the enemy team's composition and how much built-in cc they have): Damage (or health) for early game and as many potions as you can afford.

Phage: Rush this to be able to snare enemy champions. Perfect for mid-game ganking, and you don't even need to get the red buff (because it's unreliable to begin with anyway *gone after timer ends or upon death*).

Mercury Treads: I've tried Berserker Greaves and the rest, but none can compare to MT's. CC is very common in every match. The only time I'd reccomend Berserkers Greaves is when the opposing team has no cc at all!

Brutalizer: By the time you get this beauty, it's mid or late game, and you'll get a significant boost in damage and cool-down, which is much needed for Nocturne.

BlackCleaver: Combined with your ArP from runes and your Brutalizer, it's time to chop up those squishies, hopefully taking their armor into the negatives (if they have no added armor and aren't super-beefy tanks... for that you'd want Madreds instead of this).

Frozen Mallet: Now you'll slow down enemy champions 100% so long as they don't block you. Once you "R" into an enemy champion, he ain't going no where. And if he flashes, well you have flash too! :P

Youmuu's Ghostblade: Use the active on it while engaged in fights. Frozen Mallet should keep them from escaping so that your active is fully employed and successful.

Blood Thirster: You'll want survivability at this point. You can get the Vampiric Scepter a little bit earlier if you feel you're having trouble staying alive out there. You'll stay out of base a lot longer than normal with this bad boy fully assemblede!

Banshee's Veil: Combined with Nocturne's built-in spell shield, this item makes you withstand twice the **** while wailing on those casters until they're beyond dead.

Max your Q as soon as possible (your main source of skill damage and movement speed), and then your E (for the fear) If using this build. Your W is last simply because it scales pretty low, and you'll have items that give you the AS you need.

I'd advise you not to deviate from this set-up. You can do whatever you like, but don't down-rate my build because of it. Use my build the way I prescribed and play smart, and you should come out on top.

Nocturne benefits best from Ghost, Exhaust, and Flash. All other summoner spells aren't really as great for this AD champion. They'll work, but to me they're simply sub-par. Just don't take Revive! >=P ~~~

Play smart! During early game you'll want to let your laning partner know to let the enemy champs push minions near your turret so that you can last-hit some minions there and minimize the thread of being harrassed. There are some crazy players out there that'll try to poke at you with their ranged skills, but if you play smart, they'll eat turret hits or get into close enough range for a very mean assault, or possibly even a first blood or kill. Good luck, and let the games begin! =D