This is just a basic build for Nocturne.

Please do NOT take into consideration what the stats are at level 18 on the guide... Since the guide contains far to many Marks and Quintessences and the items are final and dependent on what you are doing the stats are not really well calculated...

ANTI CASTER (Berserker's Greaves and Trinity Force for Movement - 436.8/only differences presented):

The Hexdrinker item set grants:
Health - 2394.96
Mana - 1095
Health Per 5 sec - 20.14
Mana Per 5 Sec - 14.48
Armor - 110.5
Magic Resist - 112.5
Damage - 308.98
Critical Chance - 35
Ability Power - 30

The Banshee's Veil item set grants:
Health - 2769.96
Mana - 1470
Magic Resist - 132.5
Damage - 273.98

ANTI TANK (Berserker's Greaves and Trinity Force for Movement - 436.8/only differences presented):

The Madred's Bloodrazor item set grants:
Health - 2394.96
Mana - 1095
Health Per 5 sec - 20.14
Mana Per 5 Sec - 14.48
Armor - 135.5
Magic Resist - 52.5
Damage - 343.98
Critical Chance - 35
Ability Power - 30

The Youmuu's Ghostblade item set grants:
Armor - 110.5
Armor Penetration - 20
Critical Chance - 50

SURVIVAL SET (Berserker's Greaves and Trinity Force for Movement - 436.8):
Health - 3314.96 (3764.96 with full stack Warmog's Armor)
Mana - 1095
Health Per 5 sec - 50.14 (65.15 with full stack Warmog's Armor)
Mana Per 5 Sec - 14.48
Armor - 155.5
Magic Resist - 52.5
Damage - 273.98 (349.27 with full stack Warmog's Armor and Atma's Impaler)
Critical Chance - 53
Ability Power - 30

For the Boots of Swiftness and Phantom Dancer please make calculation yourself.

The Seals and Glyphs are quite straight forward set, but as for the Marks and Quintessences you will need to make a choice for ether Damage or Speed...

It is up to you...

The Offense ones are simply to boost all possible physical damage, not ability power...
Damage output, armor penetration, attack speed, the works...

The Utility masteries are simply for a fast progression... Except for Haste...
Less time dead and more experience can make a difference...

Again... Quite straight forward...

Here it gets tricky...
Duskbringer: Try to grab this one at lvl 1 for jungle or harassment... Can even be used as escape mechanism in lane...
Shroud of Darkness: If you go for a jungle start you will need this for the passive speed bonus... If you go in lane the active component might make the difference rather then the speed... This one might just save your life here and there...
Unspeakable Horror: Quite useless early on so no real need for it... I set it for lvl 5 because you might just need it at 6 when you go hunting...
Paranoia: Grab this at each level you can... It is THE hunting skill for you... Good for KS, entering combat and hunting...

This one is quite tricky... Hold on...

Boots of Swiftness vs Berserker's Greaves: There pure movement speed or speed on both sides...
Wriggle's Lantern and Infinity Edge: Quite basic... No choice to make...
Trinity Force vs Phantom Dancer: Make a choice... Depends on game stile... Again, great movement speed boost vs overall bonuses...
Wit's End, Hexdrinker and Banshee's Veil: Anti caster... All are good choices here... Chose...
Last Whisper and Madred's Bloodrazor: Anti health hero hunter...
Last Whisper and Youmuu's Ghostblade: Anti armor/overall tank hunter...
Atma's Impaler and Warmog's Armor: Good overall survival and damage combo...

It really is all about how you play, against who you play and what your team is doing...