DARK SOULS II

You'll need to either purchase or find all hexes available in-game. You can purchase them from various merchants in-game and/or find them spread throughout the world, the hexes you'll need are

Dark Orb – Bought from Straid, Felkin, or Grandahl
Dark Hail – Bought from Straid, Magerold, or Grandahl
Dark Fog – Bought from Straid
Affinity – Bought from Straid
Dead Again – Bought from Chloanne
Dark Weapon – Bought from Felkin
Whisper of Despair – Found in the Grave of Saints. Use a Pharos Lockstone to make a drawbridge come down.
Repel – Bought from Straid. Requires the Giant Lord soul.
Twisted Barricade – Found in the Doors of Pharros
Numbness – Bought from Straid. Requires the Old Dead One's soul, which is dropped from the Rotten on new game plus or after using an ascetic.
Scraps of Life – Bought from Cromwell
Darkstorm – Bought from Magerold or Grandahl
Resonant Soul – Pilgrims of Dark Rank 1 (See Selfless Giver)
Great Resonant Soul – Pilgrims of Dark Rank 2 (See Selfless Giver)
Climax – Pilgrims of Dark Rank 3 (See Selfless Giver)
Resonant Flesh – Bought from Felkin
Resonant Weapon – Bought from Felkin
Lifedrain Patch – Bought from Straid. Requires Darklurker soul. (See Selfless Giver)
Profound Still – Bought from Grandahl

Credit for full list to Lonesheep over pstrophies.org

This one will take you at the very least a go through the Black Gulch with the Hidden Bonfire there on Bonfire Intensity 2(or NG+), but other than that it is actually possible to get this on your very first playthrough. Despite that, you'll be doing one of the hardest optional bosses and hit Rank 3 with a covenant trying to get this so there's your bonuses for doing this!

A quick reference where to find the merchants that deserve some extra explanation; Straid of Olaphis is in the Lost Bastille at the very top floor near the bridge that leads to Sinners Rise, easiest reached from the Saltfort bonfire at the Sinner's Rise or by going up from the Servant's Quarters. Initially he's petrified so grabbing the nearby Fragrant Branch of Yore found in Sinner's Rise in the submerged jailcells is necessary to make him his lively self again.

Felkin the Outcast sits in a chair facing a wall in the cavern path leading up to the Huntman's Copse near the Undead Refuge bonfire, it takes 8 Intelligence and Faith to be able to buy items from him.

Darkdiver Grandahl is in 3 locations around the world, in Drangleic Castle from the King's Gate bonfire, the large room with the Sentinel Ruin minibosses, the far left door has a hole leading to a bonfire with him nearby, he is also in Shaded Woods past the two guard towers that lead to the boss, you can find a very small boarded up area in the grass, standing on it will drop you into the earth where you'll find him again. His last area is arguably the most difficult to find, first you got to get the Forgotten Key from the Giant Elites in Black Gulch, they're in a hidden area that you can access from a small ledge you can fall onto. Use this video to track the area down as it's damn near impossible to explain in text. Once you find him at any one of the three locations, join his covenant and you'll be able to buy his spells.


PURCHASABLE
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DARK ORB - 600 souls, Straid of Olaphis, Felkin the Outcast, Darkdiver Grandahl
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Easiest gotten from Felkin, I'd say.

DARK HAIL - 1500 souls, Straid of Olaphis, Magerold of Lanafir, Darkdiver Grandahl
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Magerold is in the Iron Keep, from the beginning bonfire go across the bridge and kill the two Alonne Knights, keep right when you head inside and you'll find sir sitting in the corner tucked away nicely. Watch your back incase one of the nearby Alonne Knights aggro'd.

DARK FOG - 5200 souls, Straid of Olaphis
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This one can also be found in the Gutter, in one of the jars you can smash at the very bottom right before heading into the Black Gulch.

AFFINITY - 11500 souls, Straid of Olaphis
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Can also be gotten from the little goblin looking guys in Things Betwixt and Aldia's Keep, very rarely though so really it is just better going for the 11,5K purchase.

DEAD AGAIN - 4000 souls, Chloanne, the Stone Trader
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Chloanne will be at Majula once you speak to her and exhaust her dialogue until repetition in Harvest Valley, she's to the right from the Poison Pool bonfire, can't miss her.

DARK WEAPON - 2700 souls, Felkin the Outcast
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Unique spell to Felkin.

SCRAPS OF LIFE - 2200 souls, Cromwell the Pardoner
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Cromwell is found ontop of the church where you defeated Prowling Magus and Congregation in Brightstone Cove Tseldora, the ladder is on the right side of the door if you come from the nearby bonfire, left side if you come from the boss room.

It can also be found in a hidden alcove in the Black Gulch by jumping off a ledge near the first bonfire before reaching the first set of tar pools, use a Torch to be able to clearly see the ledge.

DARKSTORM - 4700 souls, Magerold of Lanafir, Darkdiver Grandahl
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Can be purchased from either one.

RESONANT SOUL - 1100 souls, Felkin the Outcast
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You can either buy this one for the low amount of souls at Felkin or get it rewarded automatically from reaching Rank 1 in the Pilgrims of the Dark by completing one of the Dark Chasms near Grandahl.

GREAT RESONANT SOUL - 3400 souls, Felkin the Outcast
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Basically the same principle of Resonant Soul, just either cough up the 3400 souls at Felkin or complete the all three Dark Chasms of the Abyss and hitting Rank 2 in Pilgrims of the Dark.

RESONANT FLESH - 3400 souls, Felkin the Outcast
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Again a spell unique to Felkin.

RESONANT WEAPON - 4000 souls, Felkin the Outcast
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One of the last unique spells from Felkin.

PROFOUND STILL - 13000 souls, Darkdiver Grandahl
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Spell unique to Grandahl, purchase it from him.


FROM BOSS SOULS
Straid of Olaphis is the one who turns boss souls into spells, however his Trade inventory doesn't show if you already own the spells so double check these before swapping souls. It also always has a base cost of 5000 souls to trade souls for spells.
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REPEL - Giant Lord Soul
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Soul obtained from killing the Giant Lord in the Memory of Jeigh near the end of the game.

LIFEDRAIN PATCH - Darklurker Soul
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You get this one from killing the Darklurker, the boss is available after completing all three Dark Chasms of the Abyss areas and returning into any one of three, reaching the end and falling into the boss room.

NUMBNESS - Old Dead One's Soul
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Obtained from the Rotten when the bonfire intensity at the Hidden Chamber bonfire is at 2 or higher, either by burning a Bonfire Ascetic or by reaching New Game Plus. You'll get both Soul of the Rotten and Old Dead One's Soul once he dies.


FOUND IN THE WORLD
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WHISPERS OF DESPAIR - Grave of Saints, near Harval's Resting Place bonfire
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You'll need a Pharros Lockstone for this one, from the bonfire mentioned above trek on through the area as normal. When you reach the area with raised drawbridges, put a Lockstone into the slot right infront of the bridge(it's the one nearest to from where you came), lower the bridge and head into the area.

Follow the circular stairs up and the item is on a corpse on the first tomb on the left.

TWISTED BARRICADE - Doors of Pharros, near Gyrm's Respite bonfire
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Head into the main Pharros area leading to the Royal Rat Authority boss and proceed to make your way to the boss. In the far back, you need to go up a ladder to carry on, up there on the left is a chest with the spell and a Faintstone in it.


FROM COVENANTS
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CLIMAX - Pilgrims of the Dark, Rank 3
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You'll get Resonant Soul and Great Resonant soul from Ranks 1 and 2 respectively by shutting down all three Abyss Chasms. Climax you'll get when Darklurker dies and you go back to talk to Mr. Grandahl. Along with Climax, you'll get the entire Xanthous Set and a Dragon Chime.

And that should be all of 'em!