Nocturne is all about attack speed and damage and if you have those two why not get life steal as well. How I use him is to use Paranoia and chose the person who is going to go back to the summoning pool. Then u want to use dusk bringer on the target and in front of where their going and unspeakable horror on them so when they try to run they get feared(if the person you choose is a caster or someone with some cc your will need to hit shroud of darkness while jumping towards them).
Nocturne can be used as an awsome laning character as i have shown in this guide but also as an amazing jungle character by getting to level 6 and diving in on people if the over extend with an amazing chase in dusk bringer and a fear in unspeakable horrer.