Not Updated For Current Season
Nocturne is a great assasin type champion for both 3v3 and 5v5. Currently, I play Nocturne as a heavy attack damage character. I'm here to share my knowledge with you, my fellow players.
I will try to keep this guide as updated as I can, depending on if you guys like it.
I choose Critical Chance (Malice) Marks, Seals, Quints, and Attack Speed (Alacity) Glyphs.
The crit runes allow nocturn to dominate the lane and jungle with the lantern, the critical strike combined with lantern's passive deals massive amounts of damage to minions and jungle creep. This allows you to outlevel your enemies and easily kill them later on. I believe that instead of buying items like Phantom Dancers to get crit chance, you could just use these runes.
Attack speed glyphs help you in early game fighting and leveling. Nearly 10% attack speed boost from the runes and masteries, which is not that bad.
Works well on Nocturne as it allows him to deal out a lot of damage.
Allows you to stay at the same levels as everyone else even though you will be ganking most of the time. And the extra buff time really packs a punch.
Wriggle's Lantern - Provides a good amount of defence and damage. Allows you to jungle very effectively and place free wards. An overall good item for Nocturne. Oh, and did I mention it gives you life steal?
Mercury's Treads - Gives you a fair amount of magic resistance and makes most incoming crowd control effects (stuns, fears, etc...) much shorter in duration.
Infinity Edge - Crazy item, allows you output insane damage mid-late game. Combined with the Critical Chance runes and this item makes you unstoppable.
The Bloodthirster - Very good item, furthermore making you a heavy damage dealer. Allows you to stack up good damage and life steal. Increasing your survivability.
Frozen Mallet - The passive from this item will not any running enemies get away from you as your auto-attacks will slow them. Makes you a beefy target to kill too.
Last Whisper - Gives very good armor penetration stats. Usually people will get tired of you killing them and they will stack armor. This item will help you kill those who try and oppose you.
These items are optional but might help you win the game.Berserker's Greaves - If the enemy team is not crowd control-heavy you can use this item to make Nocturne deal out even more damage.
Banshee's Veil - A great item to counter combo casters such as Annie, Anivia, LeBlanc, etc.. Blocks one negative spell completely, now isn't that great?