Well, better late than never - reposting the existing designs here and soontm I will post the ones I missed.
Colorless and monocolored
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Icon of Emptiness
Artifact Creature - Demon (U)
Flying
Icon of Emptiness can't attack or block if there are five or more colored permanents on the battlefield.
: Permanents target player controls become colorless until end of turn.
6/6
Corrupted Reformist
Creature - Demon (R)
Vigilance
At the beginning of your upkeep, untap all permanents your opponents control.
: Corrupted Reformist gets +1/+2 until end of turn.
"Once I took from the weak. Now I give to them the gift of second chances."
4/4
Master of the Drowned
Creature - Demon (R)
Prowess, hexproof, islandwalk
Whenever a creature dies, return target instant or sorcery card from your graveyard to your hand. If you can't, put the top five cards of your library into your graveyard. When you cast a spell that shares a name with the card you returned in this way this turn, put a +1/+1 counter on Master of the Drowned.
2/3
Glutboar
Creature - Demon Boar (C)
"Fly? Why should I? Plenty of delicious souls down here."
6/7
Jokulskolli
Snow Creature - Demon (R)
Flying, menace
Whenever a land card is put into a graveyard from anywhere, Jokulskolli deals 3 damage to each nonsnow creature, each nonsnow planeswalker, and each player and you add to your mana pool.
: Jokulskolli gains indestructible until end of turn.
4/4
Jungle Enforcer
Creature - Demon (R)
Trample
At the beginning of each opponent's upkeep, Jungle Enforcer gets +X/+X until end of turn, where X is the number of nonbasic lands that player controls, then Jungle Enforcer fights target creature that player controls.
3/3
Artifact Creature - Demon (U)
Flying
Icon of Emptiness can't attack or block if there are five or more colored permanents on the battlefield.
: Permanents target player controls become colorless until end of turn.
6/6
Corrupted Reformist
Creature - Demon (R)
Vigilance
At the beginning of your upkeep, untap all permanents your opponents control.
: Corrupted Reformist gets +1/+2 until end of turn.
"Once I took from the weak. Now I give to them the gift of second chances."
4/4
Master of the Drowned
Creature - Demon (R)
Prowess, hexproof, islandwalk
Whenever a creature dies, return target instant or sorcery card from your graveyard to your hand. If you can't, put the top five cards of your library into your graveyard. When you cast a spell that shares a name with the card you returned in this way this turn, put a +1/+1 counter on Master of the Drowned.
2/3
Glutboar
Creature - Demon Boar (C)
"Fly? Why should I? Plenty of delicious souls down here."
6/7
Jokulskolli
Snow Creature - Demon (R)
Flying, menace
Whenever a land card is put into a graveyard from anywhere, Jokulskolli deals 3 damage to each nonsnow creature, each nonsnow planeswalker, and each player and you add to your mana pool.
: Jokulskolli gains indestructible until end of turn.
4/4
Jungle Enforcer
Creature - Demon (R)
Trample
At the beginning of each opponent's upkeep, Jungle Enforcer gets +X/+X until end of turn, where X is the number of nonbasic lands that player controls, then Jungle Enforcer fights target creature that player controls.
3/3
Allied bicolor
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Prakasur, Twister of Rays
Legendary Creature - Demon Wizard (M)
Vigilance
: Prakasur, Twister of Rays deals 2 damage to target attacking or blocking creature and 1 damage to you.
: Counter target spell with converted mana cost 2 or less or target activated or triggered ability from a source with converted mana cost 2 or less. Prakasur deals 1 damage to you. (Mana abilities can't be targeted.)
: Creatures you control get +0/+2 until end of turn. Prakasur deals 1 damage to you.
4/4
Flaying Puppeteer
Creature - Demon (R)
Flying
: Create a token that's a copy of target creature. Sacrifice that token when it enters the battlefield.
"Dance your heart out. It won't be for long."
3/4
Keeper of Widespread Ruin
Creature - Demon (M)
Flying, haste, deathtouch
Pay half your life, rounded up: Destroy target creature or planeswalker. That permanent's controller loses 1 life. Create a 1/1 red Devil creature token. It has "When this creature dies, it deals 1 damage to target creature or player."
"Fall."
5/4
Abyzouth of the Hunt
Creature - Demon (R)
Flying, first strike, deathtouch, prowess, trample
"Why must my pretties flee from me? Then again, the struggle makes their meat all the sweeter."
3/2
Blessingmelter
Creature - Demon (R)
Flying
At the beginning of your upkeep, sacrifice an enchantment. If you do, create an X/X green Ooze creature token, where X is twice that enchantment's converted mana cost. If you can't, sacrifice Blessingmelter.
Eminently practical, they tend not to see much use for metaphysical boons.
4/5
Legendary Creature - Demon Wizard (M)
Vigilance
: Prakasur, Twister of Rays deals 2 damage to target attacking or blocking creature and 1 damage to you.
: Counter target spell with converted mana cost 2 or less or target activated or triggered ability from a source with converted mana cost 2 or less. Prakasur deals 1 damage to you. (Mana abilities can't be targeted.)
: Creatures you control get +0/+2 until end of turn. Prakasur deals 1 damage to you.
4/4
Flaying Puppeteer
Creature - Demon (R)
Flying
: Create a token that's a copy of target creature. Sacrifice that token when it enters the battlefield.
"Dance your heart out. It won't be for long."
3/4
Keeper of Widespread Ruin
Creature - Demon (M)
Flying, haste, deathtouch
Pay half your life, rounded up: Destroy target creature or planeswalker. That permanent's controller loses 1 life. Create a 1/1 red Devil creature token. It has "When this creature dies, it deals 1 damage to target creature or player."
"Fall."
5/4
Abyzouth of the Hunt
Creature - Demon (R)
Flying, first strike, deathtouch, prowess, trample
"Why must my pretties flee from me? Then again, the struggle makes their meat all the sweeter."
3/2
Blessingmelter
Creature - Demon (R)
Flying
At the beginning of your upkeep, sacrifice an enchantment. If you do, create an X/X green Ooze creature token, where X is twice that enchantment's converted mana cost. If you can't, sacrifice Blessingmelter.
Eminently practical, they tend not to see much use for metaphysical boons.
4/5
Enemy bicolor
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Heavenfire Tempter
Creature - Demon (M)
Flying, double strike, menace
When Heavenfire Tempter enters the battlefield, you may pay 13 life. If you do, destroy all permanents your opponents control.
3/13
Bramblereaper
Creature - Demon (M)
Indestructible
All spells, permanents, and cards that aren't on the battlefield have deathtouch.
Whenever a planeswalker is dealt damage, destroy that planeswalker.
4/1
Weather Fiend
Creature - Demon (R)
Flying
Whenever you draw a card, choose one -
• Target creature gets -1/-0 until end of turn.
• Weather Fiend deals 1 damage to target creature with flying.
• You may tap or untap target land.
3/3
Visage of Dissimulation
Creature - Demon Illusion (R)
Flash
Menace
When Visage of Dissimulation enters the battlefield, target creature gets -3/+6 until end of turn.
When Visage of Dissimulation leaves the battlefield, target creature gets +6/-3 until end of turn.
3/3
Vainglorious Greatknight
Creature - Demon Knight (R)
Trample, protection from black
Whenever Vainglorious Greatknight deals combat damage to a player, put a maneuver counter on Vainglorious Greatknight.
At the beginning of your upkeep, remove a maneuver counter from Vainglorious Greatknight. If you do, Vainglorious Greatknight deals 4 damage to target creature or player and you gain 4 life.
4/3
Creature - Demon (M)
Flying, double strike, menace
When Heavenfire Tempter enters the battlefield, you may pay 13 life. If you do, destroy all permanents your opponents control.
3/13
Bramblereaper
Creature - Demon (M)
Indestructible
All spells, permanents, and cards that aren't on the battlefield have deathtouch.
Whenever a planeswalker is dealt damage, destroy that planeswalker.
4/1
Weather Fiend
Creature - Demon (R)
Flying
Whenever you draw a card, choose one -
• Target creature gets -1/-0 until end of turn.
• Weather Fiend deals 1 damage to target creature with flying.
• You may tap or untap target land.
3/3
Visage of Dissimulation
Creature - Demon Illusion (R)
Flash
Menace
When Visage of Dissimulation enters the battlefield, target creature gets -3/+6 until end of turn.
When Visage of Dissimulation leaves the battlefield, target creature gets +6/-3 until end of turn.
3/3
Vainglorious Greatknight
Creature - Demon Knight (R)
Trample, protection from black
Whenever Vainglorious Greatknight deals combat damage to a player, put a maneuver counter on Vainglorious Greatknight.
At the beginning of your upkeep, remove a maneuver counter from Vainglorious Greatknight. If you do, Vainglorious Greatknight deals 4 damage to target creature or player and you gain 4 life.
4/3
Allied tricolor
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Dictator of the Lacquered Garden
Creature - Demon (M)
At the beginning of your upkeep, choose casting spells, playing lands, attacking or blocking with creatures, or drawing cards. You can't do the others until your next turn.
Spells you cast cost less to cast, you may play an additional land on each of your turns, and other creatures you control get +1/+1 and have vigilance.
: Draw a card.
Paradise appears as he wills it to.
3/3
Infernal Patron
Creature - Demon (R)
Whenever a creature enters the battlefield under your control, you may untap Infernal Patron.
: When another target creature you control dies this turn, you may pay life equal to that creature's toughness. If you do, draw that many cards. Otherwise, you gain life equal to that creature's power.
: Target creature gets +1/+1 or -1/-1 until end of turn. Untap that creature.
"You have much to learn. Almost as much as I have to teach you."
4/5
Scroll-Stare Fiend
Creature - Demon (R)
Haste, menace
Whenever you search a library, you may reveal the cards you searched for. If you do, Scroll-Stare Fiend gets +X/+0 until end of turn, where X is the total converted mana cost of cards revealed in this way.
3/2
Abomination of Devastation
Creature - Demon Horror (R)
Reach, deathtouch
When Abomination of Devastation enters the battlefield, choose one or both -
• Destroy all artifacts, all Artificers, and all Dwarves.
• Destroy all enchantments, all Druids, and all Elves.
7/7
Fiery Juror
Creature - Demon Avatar (R)
Trample
All creatures able to block Fiery Juror do so.
Whenever Fiery Juror attacks, defending player gains 5 life.
5/5
Creature - Demon (M)
At the beginning of your upkeep, choose casting spells, playing lands, attacking or blocking with creatures, or drawing cards. You can't do the others until your next turn.
Spells you cast cost less to cast, you may play an additional land on each of your turns, and other creatures you control get +1/+1 and have vigilance.
: Draw a card.
Paradise appears as he wills it to.
3/3
Infernal Patron
Creature - Demon (R)
Whenever a creature enters the battlefield under your control, you may untap Infernal Patron.
: When another target creature you control dies this turn, you may pay life equal to that creature's toughness. If you do, draw that many cards. Otherwise, you gain life equal to that creature's power.
: Target creature gets +1/+1 or -1/-1 until end of turn. Untap that creature.
"You have much to learn. Almost as much as I have to teach you."
4/5
Scroll-Stare Fiend
Creature - Demon (R)
Haste, menace
Whenever you search a library, you may reveal the cards you searched for. If you do, Scroll-Stare Fiend gets +X/+0 until end of turn, where X is the total converted mana cost of cards revealed in this way.
3/2
Abomination of Devastation
Creature - Demon Horror (R)
Reach, deathtouch
When Abomination of Devastation enters the battlefield, choose one or both -
• Destroy all artifacts, all Artificers, and all Dwarves.
• Destroy all enchantments, all Druids, and all Elves.
7/7
Fiery Juror
Creature - Demon Avatar (R)
Trample
All creatures able to block Fiery Juror do so.
Whenever Fiery Juror attacks, defending player gains 5 life.
5/5