Q-Jei

After seeing the tentative of @Vince&Sylveon conceptualizing custom field effects, I couldn't resist to do so. Here comes mine 

Devil's Den Field

"The world is finite, our hopes spill over its rim."

General Effects

"Abyss favors the forsaken!" (Dark and Ghost)
"Fire and brimstone accompany your strength!" (Fire)

"The hot air mass from the blazing depths softened the attack!" (Water and Ice)

"The infertility of the soil withered the attack!" (Grass)

"Your emotions keep you away from power!" (Fairy and special Normal moves)

Normal and Fairy types are now weak to Dark moves.
Normal types can be hit by Ghost moves.
Dark and Ghost moves increase in base power by 1.5x.

Fire moves increase in base power by 1.1x and hit Fairy type super effectively.

Ice and Water moves decrease in base power by 0.9x.

Grass moves from grounded Pokemon decrease in base power by 0.8x.
Fairy and special Normal moves decrease in base power by 0.5x.
Ground moves from grounded Pokemon deal additional Fire damage.
Sleeping Pokémon that are not Ghost or Dark types, or have Wonder Guard, lose 1/8 of their max HP each turn.

Pokémon that flinch have their Speed reduced by one stage.
Prevents all weather and generated Field Effects.

Abilities Affected

Pokemon with Shadow Shield take x0.5 decreased damage.
Dark Aura double the damage dealt by Dark moves and ignores Aura Break.
Pokemon with the ability Multitype or RKS System become Dark type at all times.
Cursed Body will always disable the move of the attacking Pokemon for 4 turns.

Pressure deducts 3 PP instead of 2 whenever PP would normally be deducted.
Prankster increases the priority of all the moves by one.

Justified, Pixilate and Victory Star have no effect.

Moves Affected

Moves that will always fail

Aurora Veil

Cosmic Power

Draco Meteor

Floral Healing

Meteor Mash

Moonblast

Moonlight

Morning Sun

Nature Madness

Solar Beam

Solar Blade

Swift

Synthesis

Wish

Moves whose base accuracy becomes 100

Dark Void

Hypnosis

Inferno
Will-O-Wisp

Moves with a 1.3x increase in base power

"Devilish forces take over the attacker's gesture!"

Dark Pulse

Extrasensory

Fake Out
Foul Play
Knock Off

Mystical Fire
Night Slash
Oblivion Wing

Phantom Force
Shadow Ball
Shadow Claw
Shadow Force

Shadow Punch
Shadow Sneak
Spectral Thief
Sucker Punch

Moves with a 1.5x increase in base power 

"The bowels of the underground are rebelling against their inner nature!"

Ancient Power

Blast Burn
Blue Flare

Continental Crush
Earth Power
Earthquake
Eruption

Fire Spin

Fusion Flare

Lava Plume

Magma Storm

Sacred Fire

Tectonic Rage

Other Moves

If used by Non-Ghost-type Pokemon, Curse boosts Attack and Defense by two stages and doesn't decrease Speed.
If used by Ghost-type Pokemon, Curse applies a random status effect to the target in addition to its usual effects and doesn't hurt the user.
Doom Desire gains a 4x increase in base power and deals additional Fire damage.

Inferno doubles in power in addition to to the previous 1.1x boost it receives from being a Fire move and deals additional Dark damage.
Light That Burns the Sky decreases in base power by 0.666x.

Swords Dance will boost Attack by three stages.
Nasty Plot will boost Special Attack by three stages.

Calm Mind will boost Special Attack and Special Defense by two stages.

Scary Face lowers the target's Attack, Special Attack and Speed by two stages.

Feint Attack increases in power by 1.5x in addition to to the previous 1.5x boost it receives from being a Dark move and boosts Evasion by one stage.

Hex increases in power by 1.5x in addition to to the previous 1.5x boost it receives from being a Ghost move and confuses the target.

Ominous Wind increases in power by 1.5x in addition to to the previous 1.5x boost it receives from being a Ghost move and has a 50% chance of raising the stats of the user (from 10%).

Dream Eater increases in base power by 1.666x and up to 66% of the damage dealt to the target is restored to the user as HP (from 50%).

Night Shade increases in base power by 1.666x and may make the target flinch.

Power Gem doubles in power and lowers the accuracy of the target by one stage.

Smack Down doubles in power and deals additional Fire damage.
Scald and Steam Eruption increase in base power by 1.666x (totaling 1.5x).

Magma Storm and Fire Spin both deal 1/6 of the target’s HP each turn (rather than 1/8) and confuse the target.

Spite cuts 6 PP of the target's last move (from 4).
Mind Reader and Miracle Eye will boost Attack and Special Attack by two stages but confuse the user in addition to their usual effects.
Nightmare will boost Attack and Special Attack by two stages in addition to its usual effects.

Taunt lasts six turns instead of three.

Perish Song lasts one turn instead of three.
Black Hole Eclipse and Never-Ending Nightmare damage increases x4.

Judgement and Mind Blown will inflict damage equal to the target's current HP but cause the user to faint after use.
Nature Power will become Inferno.
Secret Power may induce sleep or burn.
Camouflage will turn the user into the Dark type.

Transitions to other Fields

Judgement will permanently terminate this field effect.

Seed

Magical Seed boosts Attack, Special Attack and Speed but applies Torment to the user.