Hey this is my first build. I've played Nocturne atleast 75% of my games and have tweaked this build from what people told me to what works for me. This ends out with you doing over 600+ damage to minions, 175ish to buildings and 200-500 to champs. Going after all the nukes is your best bet because of the two ability blocks you have and how fast you go through health. So yeah you could die in team battles quick if the other team is smart and goes for you first but thats why I'm rolling ghost and flash to get away. Ghost also helps you get around the map quick if something is going on that you need to help with.

I have armor penetration for early fights with tanks, attack speed to get you to 1 attack speed before you even leave the platform, and mana/health so you can stay in lanes early game and feed easier.

Going an easy 21/0/9 build for agressive game play. you need the extra experience and gold for the whole game or you will get left behind by everyone else. The rest is for getting DPS maximized, which really matters for you to get the kills in 1v1 fights, which you should stick to.

[Dagger] Gets you to 1 attack speed before you get off platform.
[Health Pot] Lets you stay in lanes early.
[Berserker's Greaves] For the extra attack speed and it makes it way easier to cath squishy champs that bolt when their health drops as fast as you cause damage.
[Zeal] Of the three choices in making Phantom Dancers, this is best choice to get if you can only afford one. If you can get two get the most expensive one you can afford at that time.
[Pantom Dancer] Easy choice for Nocturne because movement speed, crit chance and attack speed, what he's made for.
[Catalyst the Protector] Just what to get first for Banshee's Veil.
[Banshee's Veil] Nocturne needs this to waste a reaction move used by the enemy you are rushing. Also good for taking down nukes that have strong initial attacks.
[Last Whisper] Now that you have this you can actually take down tanks faster than they will think you can. And since they usually cant deal massive damage you should win all those fights before they know to get out.
[The Black Cleaver] Once you have this you become one of the fastest killers on the field. You will start dealing the massive damage that Nocturne is made for. Some might say its better to go for The Bloodthirster, but dying when you have max stacks feels like **** -.-
Now you have a choice. Most likely by this point the game will be over so this next part might not even matter. But depending on how your games going this choice will affect how much you live and how much you kill.
[Madred's Bloodrazor] This is a good choice if the game is moving to the enemies winning. Extra armor and attack to help the fights go in your favor but let you last longer.
[Lich Bane] This will help you chase down people easier. More movement speed and ability power, your duskbringer will be much more effective now and 1v1 fights will go much quicker.

You might hate on me for this but you are made to ks, your team will hate you but 1 move is ghost with benifits for chasing and 1 move literally teleports you to the enemy to pick up the running champ just out of your reach. So in team fights go for low health enemies and leave the bulky enemies with 3000+ health to everyone else.