Nocturne is a melee DPS assassin who is good at clearing out minions and chasing down low health champions.In my build, the Nocturne you will be playing is a tanky yet quick assassin that focuses on chasing down and finishing off champions, sometimes turret diving. His 3k+ health, high attack speed and nearly 500 movement speed make him perfect for the job.

"Nocturne had drifted through the minds of humans for endless expanses of time. He beheld the faces they yearned for, absorbed the desires that coursed through their veins, and inhabited the fantasies that set their hearts ablaze. He lingered within the despicably human illusions they conjured in the narrow expanse that was their puny brains.
From all of them, he consumed the magic that rippled through their souls."[/i]
- The League Judgement

The summoner spells I have recommended are at the top, with exhaust and heal being my choices.

Recommended Spells:

Exhaust - Reduces a target's Attack Damage by 70%, Ability/Item damage by 35% and slows its Movement Speed by 40% for 3 seconds. Great for chasing enemies/running away from them. Use it when they are caught by a dusk trail for maximum effect.

Heal - Restores 140 + (lvl x 20) Health to your Champion and 50% of that to nearby allies. All targets healed will have a 50% reduction in heal amount from other summoner heal spells for 20 secs. AoE range: 600. Really useful with nocturne – as with my build he is used to turret dive, heal is good for these purposes.

Replacement Spells:

Ghost - Your Champion ignores unit collision and moves 27% faster for 10 seconds. Helps when you need to escape/get somewhere fast.

Teleport - After 4 seconds, teleports your champion to target friendly minion or turret. 3 minute cooldown if cancelled. Useful to go assist friends/a faster replacement of recall.

Cleanse - Removes all control debuffs, silences and blinds from your champion and reduces the duration of subsequent stuns, slows, taunts, fears, snares, sleeps, and immobilizes by 65% for the next 3 seconds. No character is immune to CC.

Flash - Teleports your Champion to target nearby location under your mouse cursor. Useful, because Nocturne is a DPS melee assassin – popping up behind them, catching then with a dusk trail and throwing a shield can hit them good.

Ignite - Targets a single champion dealing 50 + (lvl x 20) true damage over 5 seconds. Also reduces the target’s healing and regeneration by 50%. Great for whittling down enemy health.

Clarity - Restores 160 + (lvl x 30) Mana to your champion and 50% of that to nearby allies. Useful for staying on the lane for ages, but if you need it then you’re playing wrong.

Useless Spells:

Revive - Instantly revives your Champion at your Spawning Pool, increasing your movement speed by 225%, diminishing to normal over 12 seconds. 9 minute cooldown. Easier to wait for it, rather than wasting a slot.

Smite - Deals 420 + (lvl x 25) true damage to target enemy minion or pet. Even though Nocturne is used as a jungler sometimes, smite isn’t useful.

Fortify - Grants invulnerability to all Allied Turrets and causes them to attack 100% faster for 6 seconds. While Fortify is ready to cast, you deal 9 bonus damage to minions. Let someone else waste their spell on it.

Rally - Summons a beacon with 200 + (lvl x 25) Health for 15 seconds which increases allied Attack Damage by 10-35. Tell me, why does no one else use it?

Clairvoyance - Reveals an area of the map for 6 seconds. You'll end up wasting it trying to find the right place to cast it.

The Skills and how to use them:

Umbra Blades - Every 10 seconds, Nocturne's next attack strikes surrounding enemies for 120% of his physical damage and heals himself for 15 / 20 / 25 per target hit. Nocturne's autoattacks reduce this cooldown by 1 second. Great for clearing out minion waves – with high damage and healing, this truly is a good passive

Duskbringer - Nocturne sends a shadow blade in a line that deals physical damage to all enemies it passes through, leaves a Dusk Trail on its path, and causes champions who are hit to also leave a Dusk Trail when they move for 5 seconds. While on the trail, Nocturne ignores unit collision and has increased movement speed and attack damage. Nocturne’s main ability during combat – can be used to hit multiple enemies over a long distance, or to use when giving chase to a low health enemy. Sometimes is also useful for running away.

Shroud of Darkness - (Passive): Nocturne empowers his blades, permanently gaining attack speed.
(Active): Nocturne creates a magical shield for 2 seconds which can block the next enemy spell that hits him. If an ability is blocked by the shield, his passive attack speed is doubled for 5 seconds. This ability is a blessing if used properly. And to use it properly, it’s all about timing. Too early, when the attack hits, you’ll get hit. Too late, and you’ll have a spare shield at your hands. Use when you see an enemy approaching towards you.

Unspeakable Horror - Nocturne plants a nightmare into his target's mind, dealing magic damage over 2 seconds and placing a leash on the target for the duration. If Nocturne stays within leash range of the target for the full duration, the target becomes feared. Use this ability when you’re closely on the chase, or simply when you need to lower health. The fear debuff means if you are being chased you can grapple onto them and then they cannot control their movements, letting you escape.

Paranoia - Nocturne reduces the sight radius of all enemy champions and removes their ally vision for 4 seconds; this includes champions, minions, wards and towers. While Paranoia is active, Nocturne can launch himself at a visible enemy champion, dealing physical damage. A great finishing move/support move; due to the launch, you can fly towards them when they run away, or fly towards an enemy chasing your friend. May also be used as a hit and run. The “artificial fog of war” created is good when the enemy is planning a strategy/ ambushes.

The lines that Nocturne says in-game:
"
"Embrace the darkness."
"Afraid of the dark?"
"Are you getting tired yet?"
"Are you my nightmare, or am I yours?"
"Cross over."
"Cut them from this world!"
"Do I scare you, summoner?"
"Drip drop, the sound of blood..."
"Help is not coming..."
"I sense fear."
"People are flames to be blotted out."
"Suffer eternally!"
"Swim in terror."
"The light is fading."
"Their twilight approaches."
"This place disgusts me."
"Bring me more fleshbags to slice up!"
"Weather forecast for tonight: dark, with a chance of pain!"
"All alone..."
"Darkness..."
sinister laughter*”

Nocturne's Story:

Before Nocturne, people believed that dreams were figments of their imaginations, meaningless images that flashed through the mind when one slept. This belief was put to the test when a rash of sleep-related incidents started afflicting summoners of the League. Some would wake up screaming, terrified and beyond consolation. Some could not fall asleep, slowly going mad as the nights ticked by. Some simply never woke up. Physicians were baffled until a Field Architect happened to pass out next to a nexus on the Twisted Treeline. Witnesses said he cried out once and then stopped breathing. Immediately after, magical energy arced out from the nexus, and Nocturne appeared.
Nocturne did not take his introduction to this world kindly. He slaughtered everything he could find before summoners were able to magically confine him. After a period of intense study, League experts divined that Nocturne hunted summoners in their sleep, attacking them in a place where their magic was useless. This seemed to be his only purpose. The families of the victims demanded justice, but League officials were concerned that death might only return Nocturne to the place from which he came. They bound him to a nexus fragment, trapping him in the physical world. As punishment for his crimes, they allowed summoners to call upon Nocturne in League matches, bending his will to the summoners he hates and creating his own personal nightmare. League scholars don’t know whether he truly came from the plane of dreams, or whether there are any more like him. Some theorize that the summoning act affected the summoners subconscious minds, luring Nocturne to them in their sleep. Perhaps the most disturbing theory is that Nocturne is a person’s nightmare come to life. If this is true, they wonder, who is the dreamer?