This is how I have been building Nocturne since he came out, and it seems to work for me, so I decided to put it up to see if it would help others. I'm not the best at describing stuff, but I'll be as straight forward as I can!

Crit chance runes are amazing for Nocturne, but may also substituted with attack speed, base damage, or crit damage.

Simple masteries that help with attacking, and cooling down the time for Nocturne's skills.

Th summoner spells are pretty basic for offensive and defensive moves.

The item order can pretty much be changed after you get wit's end and berserker greaves. Also, don't forget to change out items if needed (i.e. if it's more ad then ap, then switch out banshee's veil for frozen heart).