PLEASE TRY THIS BUILD A FEW TIMES BEFORE YOU TELL ME WHAT YOU THINK OF IT
So this is my first guide ever made and I use this build when I play nocturne all the time. The runes I am willing to change because right now I have not hit level 30 and can not fully rune my rune page with tier 3 runes. However the items skill order and summoner skills are my preferred choices.

First off I like to use 9 Marks of replenishment because if you are like me and like to be aggressive with your nocturne as well as pester enemy champions then you will need a lot of mana and mana regen. Therefore high mana regen can be very useful early game. My 9 seals of alacrity are very important to me because this build will increase both damage and attack speed for nocturne and I think any early game advantage you can get will be useful therefore better attack speed will help. 9 glyphs of vigor will help during the entire game whether you have just drawn first blood or you are turret hugging while you wait for your teammate to return low health is never good so health regen can potentially save your life.

I go big into the utility tree because so much of the utility tree focuses on reducing cooldown on abilities. With Nocturne relying so much on his abilities for kills I feel it is best to have as small a cooldown as possible on each ability because I know the slight decrease has saved my life a few times. The few points into offense again help with cooldown reduction and the one point in defense gives noc a little more durability.

I always start with the berserkers greaves often buying boots of speed as the first item and getting the greaves later on. You could buy the increased attack speed instead of boots but a few times those boots have helped me to get first blood where as attack speed only did a little more damage while not securing me the kill. Next the brutalizer to start building towards yomuu's ghostblade. Third I buy the Phage to increase my attack damage while also working towards the trinity force. After buying the Phage I will upgrade my Brutalizer to Yomuu's. After that if you have been playing smartly its time to build towards Trinity Force which will alter your entire build. I have been at 2-8-13 before buying trinity force and by end game I was at 16-3-13. Trinity force is the cornerstone of this build but it will only make you that much better if you have been playing smart, you continue to play smart, and if you have Yomuu's. Next is last whisper again to upgrade AS and AD at the same time. Often here I will buy guardian angel on the off chance I get CC'd and the entire enemy team focuses on me. IMPORTANT: LATE GAME YOU WILL BE FOCUSED DO TO HIGH KILLS WITH LOW DEATHS so having GA is always a nice curve ball that could save your life. This is also where flash comes in a lot of handy. Lastly if I ever need it I will buy banshees veil, not so much for the negating an enemy ability but more for the increased health and mana. If you buy banshees for its passive then you arent using all of Nocs abilities to their fullest.

From level 1-4 I will focus on Q and E starting with Q and then E. By level 5 I will get my W. In higher level games there will be lots of people trying to play champions like Karthus or Lux or Ezreal who have ultimates that could kill you with low health. By buying W at level 5 you will always be ready for one of these abilities because W has the same effect as Banshees Veil but with less of a cooldown. At level 6 get your R also known as Paranoia because that could scavenge you so many extra kills. Once level 6 is passed I continue to level Q and E evenly because these skills are great at taking down an opponents health with the build. After 2 uses of Q and E the opponent will be ready to be taken out with paranoia if it is absolutely necessary if not repeat the use of Q and E. All adding points to W does is decrease the cooldown so wait till the last few levels to max this skill out.

I prefer to use Flash and Clarity for my summoner spells. Often my Paranoia uses will be used to tower dive an enemy champion and afterwords i need a quick way out. What better way to get out fast then to flash over a wall then use your Q and scurry away. As for Clarity this is when you pester an enemy so much that your mana regen can't bring your mana back faster than you use it.