The runes give you cooldown,Atk speed,Survivabilty, and mana regen.

Nocturne Kinda has a serious mana problem, I suggest the Clarity runes for mana regeneration.

The cool down is for his skills, his ult is the main thing.That is why I suggest cooldown for his ult.

Atk speed to give you atkspeed for some at the beginning of the game :)

Survivabilty like MR gives you the chance to survive against casters.

Starting build: What I show is his "FULL BUILD"

My build is "Dorans Blade, Berserker Grieves, Wriggle's Lantern, Phantom Dancer, Infinity Edge, Bloodthirster x3. Sell Dorans and Wriggles
Save enuff to enable you to sell Berserker Grieves to get Phantom Dancer right away(#2).
His lifesteal is 75% and his Dps is massive! He fully regenerates in 3 hits with Maxed Bloodthirsters. His Damage with baron is 520+ atk damage which is amazing! Wriggles lantern before Phantom Dancer? Because, Phantom dancer is useless without damagable crits, agree?
You can stack but where is your dmg? It allows lifesteal and to spot out some ganks.

Yes he does regenerates in 3 hits. This build enables you to 1v1 a MoFo TANK! You can probably and it is likely to 1v5 a whole team. I've done it before. :D The lifesteal is 75%, That is 3 quarters from 100%. 520+ Dps with 90% crit is amazing
That is alot of dmg! About 1300+ crit! And 75% lifesteal what a good combo.

If you're running away and still have your ult? Someone is chasing you trough the towers and dive? If you know you cannot get away. FIGHT TILL YOU DIE LIKE A SPARTAN! Use your ult against him and fear him. Atk him constantly and it is done before that I've deny someone who dived me.
Feel free to initiate with your ult on a squishy if you're smart enuff! Only if you have someone near you to assist you/kill for you. Even tho you;re the carry it wouldn't matter. If the tower hits you constantly Run or keep lifestealing

You still have a chance to kill the champ and thats where your ignite comes in.
Use that advantage to dive and ignite and run! Lifesteal your way back to a good amount of health to run away. They might target you but no longer because you are at max health!