Hi, welcome to "Nocturne easy pentakill" build (plus a little guide).
Sorry for my bad English :)
Anyway, this is a "troll" build - which means: DON'T USE THIS BUILD IN RANKED GAMES -, but it's still very powerful and, believe me, you just need the first 5 vs 5 team fight to score an easy Pentakill.
Just remember you are very squishy, so don't initiate fights but let your tank do it. Fall in in the team fight with Paranoia and already with your first basic attack ( Umbra Blades) you should be able to score a few kills. Then just chase the left targets and.. free multikills!

Pros

King of 1 vs 1;
King of Team Fights;
TONS OF DAMAGE;
TONS OF DAMAGE;
TONS OF DAMAGE;
and maybe.. also Tons of Damage.

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Cons

Squishy;
Focused;
Easily dies in 2 vs 1.

Paranoia at levels 6, 11 and 16.
Duskbringer by level 9 always.
Unspeakable Horror by level 13 and Shroud of Darkness last, but if enemy team has lots of CCs (or if they have a Karthus plus a Caitlyn, which means you need to activate your shield twice in team fights to block their ultimates).
If you need more attack speed, max Shroud of Darkness and Unspeakable Horror one level each.

MARK

Greater Mark of Desolation x 9 it's a must for Nocturne, whatever you build him.
If you really don't want Armor Penetration, which is foolish, you can buy Greater Mark of Attack Damage (recommended just for their cheapness) or you can get some Greater Mark of Attack Speed among with Greater Mark of Desolation (3x Alacrity, 6x Desolation max).

SEAL

There aren't many Seal Runes that are perfect for Nocturne, so I suggest you getting Greater Seal of Armor because of the nice bonus armor. You can also try with Greater Seal of Attack Damage or Greater Seal of Attack Speed.

GLYPH

Like Seal Runes, there aren't Glyph Runes who are made just for Nocturne. Take some Greater Glyph of Magic Resist or Greater Glyph of Scaling Magic Resist for some nice magic resist, otherwise take Greater Glyph of Attack Speed.

QUINTESSENCE

Greater Quintessence of Attack Damage. Greater Quintessence of Desolation if you prefer even more Armour Penetration, but you seriously need more Attack Damage to rock in early game.

Greater Mark of Desolation

Nocturne is a melee-assassin, so you always have to have at least 9 points in offense.
As a Jungler, I recommend you to have 21 points in Offense for great damage. The other points can be used to gain even more Attack Damage, but I won't recommend that, or you can use them on Utility for all stuff like Mana, Manax5, Speed and Relic. To be honest, when jungling I use 21|9|0, because I prefer building Nocturne more tanky, but that's up to you.
As a Laner I suggest you to pick full Attack Damage for high effectiveness. Also now a 21|9|0 mastery page can be used, to have more armour to stay longer in lane.

This whole sh*t is useless anyway, because if you want to build it full Tiamat you have to follow my f***ing build. Get a 28|2|0 and make me happy please.

Berserker's Greaves, 2 or 3 Tiamat, 1 Infinity Edge and Bloodthirster.
If you need more tanky-ness, get only 2 Tiamat and buy a Frozen Mallet and/or don't get Bloodthirster but buy a tank item (which depends if you need armour, magic resistance, life etc..).

Soon I will post a few screens and videos of me scoring multi kills and awesome scores with this build :)
Tell me what you do think about and please, remember it's a "fun" guide - don't be negative.