Make sure to utilize duskbringer and unspeakable horror in combination with your exhaust and ignite. You will have enough armor penetration to deal high damage to early game champs and get fed during the early game.

Ganking with Nocturne is perfect. Paranoia has incredible range, so if an ally champion is in trouble you will have no trouble getting to them in time to hopefully save them, and if not, you are still likely to get a kill out of it.

Duskbringer and Unspeakable Horror are not only offensive moves, but can be used as tools of escape as well. Launch your Duskbringer in the direction you want to run and use Unspeakable Horror on the chaser. If the chasing champ keeps up, he will be feared and you will get away.

Nocturne will have great lifesteal and high crit damage at the end of this build. You should be critting at around 1100. Another option that I have found to be very useful is a Nashor's Tooth instead of a second Phantom dancer. With the 25% CDR combined with the blue golem buff, you will be casting your duskbringer about everytime it ends.

Sell your brutalizer when you finish your second phantom dancer/ Nashor's Tooth, and that should help a little with your Infinity Edge.
(Just to clear up any confusion about the item sequence)

Remember that you are an assailant type, not an initiator of team fights. Ganks, and 1v1 are your specialty. This is not to say you are not useful in team fights, Just be careful.