Well, im going to start off by saying that this ubild is a little out there for nocturne,and yi but it does work most of the time...if you happen to run into a team that has lots of CC (Stuns, Roots, Fears, etc.) then you might want to consider using a different build.

Pros

+No need for attack items early game.
+Very fast building damage
+Very good ganker
+Ability to manuver the map quickly
+Team life steal

Cons

-Very squishy
-If stunned, will die
-Needs to be attacking at all times

First off, I Picked 9 Greater Mark of Desolation because you rely on your skills to supply attack damage, and the armor pen. is a very nice thing to have early game.

I picked 9 Greater Seal of Resilience because when you get to the point of having warmogs, you wont have any armor, and the armor runes will save you plenty of times when 1v1 against other dps champs.

I Picked 9 Greater Glyph of Warding for the same reason i picked the armor runes, and if your trying to chase an AP champ you will definatley want these, their range is their power, and you wanna take as little damage as possible while getting fomr point A to point B

Items! For the Nocturne build, i chose to get boots of mobility, because your sheild passivly supplys attack speed, and with those boots you'll fly around hte map like nobodys bussiness. And i chose to get Beserker's Greaves for Master Yi because well, he doesnt have an passive attack speed. I Chose to get Stark's Favor next because 1. it gives you more attack speed...and 2. because allies around you (and including you) get 20% life steal, 20% attack speel, and 30 hp per second...so this means hwen your trying to gank and Your ulti is on CD, you'll be gaining quite a bit of hp on the way there, which might just save your life. Next i chose to get Madred's Bloodrazor i did this because it supplies damage, and armor, which is helpful for a dps character. and it does 4% of hte enemys health PER HIT! So when your attack 2 times per second thats 8% hp a second just from this item alone, it will save your life in those pesky dps vs dps battles. The next item i chose to get was Bloodthirster or the Infinity Edge it depends on if your going to go with the life steal build, or the crit build, both work fine, and if your dying alot you should go for infinity edge, because there is no need to "fill" it. Next i chose to get phantom dancers, this is because it supplys more attack speed, movement speed, and osme muc hneeded crit chance, it's a very ncie addition to the build. Now, if you get far enough to get the Warmog's armor, you will be almost unkillable, once you get it "filled" to teh max, you WILL be able to solo baron, and you will also be the winning cause of most team fights, but if your stupid and think you can survive anything, charge into all of them, and die...well you cant do that..be smart and find hte right opportunity to peck them off in 2s or 1s, but once you get warmogs you will be viturally untoucable, if you play your cards right.

TEAM WORK! this is hte most important part of this build...if you dont attack WITH your team, and dont wait for your TEAM you will get mauled, over and over again. You cant win a 1v3..its not supposed to happen, and its not going to happen. So next time your hte only one alive, head my warning..and stay at the base, to defend until your teammates come alive.

Master Yi's Wuju Style and Nocturnes Dusk Bringer are both very important aspects of these builds. These are 2 of hte many few characters who's skills SUPPLY attack damage! USE THIS TO YOUR ADVANTAGE. Build attack speed! Nocturne will have a very hard time if he misses with his duskbringer, because he will be missing out on 15-35 or so extra damage per hit, so take your time with your dusk bringers and dont miss, or your going to die early game. Master yi, you only have to worry about using your e when a champion comes around, so that wont be much trouble for you.

One of hte important things about htese characters is, well they're squishy, so SIT in the bushes, dont farm minions for gold, let your partner do that while you rack up some xp, and when the enemy champion over extends to the point where you think you can kill them, launch ur duskbringer or wuju stlye and go crazy, these are the easiest kills to get, and they happen quite often, but most assassin characters dont embrace it, and well they miss the opportunity, because farming minion gold was more important.

Thats it, so go make those Teemos, Xins, Tryns, and everyother annoying as hell champ QQ and gg. Hope you liked my build, leave a suggestion and a comment on how you liked it, and please try it a few times before down rating it. thanks :)