Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control, then sacrifices a third of the lands they control. Round up each time.

Manabarbs

Whenever a player taps a land for mana, Manabarbs deals 1 damage to that player.

Dingus Staff

Whenever a creature dies, Dingus Staff deals 2 damage to that creature's controller.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Deadly Tempest

Destroy all creatures. Each player loses life equal to the number of creatures they controlled that were destroyed this way.

Decree of Pain

Destroy all creatures. They can't be regenerated. Draw a card for each creature destroyed this way.
Cycling {3}{B}{B} ({3}{B}{B}, Discard this card: Draw a card.)
When you cycle Decree of Pain, all creatures get -2/-2 until end of turn.

Spellshock

Whenever a player casts a spell, Spellshock deals 2 damage to that player.

Fire Diamond

Fire Diamond enters tapped.
{T}: Add {R}.

Read the Bones

Scry 2, then draw two cards. You lose 2 life.

Stormfist Crusader

Menace
At the beginning of your upkeep, each player draws a card and loses 1 life.

Jet Medallion

Black spells you cast cost {1} less to cast.

Dragonskull Summit

Dragonskull Summit enters tapped unless you control a Swamp or a Mountain.
{T}: Add {B} or {R}.

Charcoal Diamond

Charcoal Diamond enters tapped.
{T}: Add {B}.

Wound Reflection

At the beginning of each end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)

Havoc Festival

Players can't gain life.
At the beginning of each player's upkeep, that player loses half their life, rounded up.

Citadel of Pain

At the beginning of each player's end step, Citadel of Pain deals X damage to that player, where X is the number of untapped lands they control.

Harsh Mentor

Whenever an opponent activates an ability of an artifact, creature, or land on the battlefield, if it isn't a mana ability, Harsh Mentor deals 2 damage to that player.

Dire Fleet Ravager

Menace, deathtouch
When Dire Fleet Ravager enters, each player loses a third of their life, rounded up.

Zo-Zu the Punisher

Whenever a land enters, Zo-Zu the Punisher deals 2 damage to that land's controller.

Blasphemous Act

This spell costs {1} less to cast for each creature on the battlefield.
Blasphemous Act deals 13 damage to each creature.

Sol Ring

Barbed Wire

At the beginning of each player's upkeep, Barbed Wire deals 1 damage to that player.
{2}: Prevent the next 1 damage that would be dealt by Barbed Wire this turn.

Rakdos Carnarium

Rakdos Carnarium enters tapped.
When Rakdos Carnarium enters, return a land you control to its owner's hand.
{T}: Add {B}{R}.

Starstorm

Starstorm deals X damage to each creature.
Cycling {3} ({3}, Discard this card: Draw a card.)

Price of Glory

Whenever a player taps a land for mana, if it's not that player's turn, destroy that land.

Mogis, God of Slaughter

Indestructible
As long as your devotion to black and red is less than seven, Mogis isn't a creature.
At the beginning of each opponent's upkeep, Mogis deals 2 damage to that player unless they sacrifice a creature.

Crimson Honor Guard

Trample
At the beginning of each player's end step, Crimson Honor Guard deals 4 damage to that player unless they control a commander.

Furnace of Rath

If a source would deal damage to a permanent or player, it deals double that damage to that permanent or player instead.

Command Tower

Death Cloud

Each player loses X life, discards X cards, sacrifices X creatures, then sacrifices X lands.

Kaervek the Merciless

Whenever an opponent casts a spell, Kaervek the Merciless deals damage equal to that spell's mana value to any target.

Indulgent Tormentor

Flying
At the beginning of your upkeep, draw a card unless target opponent sacrifices a creature or pays 3 life.

Revel in Riches

Whenever a creature an opponent controls dies, create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")
At the beginning of your upkeep, if you control ten or more Treasures, you win the game.

Seizan, Perverter of Truth

At the beginning of each player's upkeep, that player loses 2 life and draws two cards.

Urborg Volcano

Urborg Volcano enters tapped.
{T}: Add {B} or {R}.

Fraying Omnipotence

Each player loses half their life, then discards half the cards in their hand, then sacrifices half the creatures they control. Round up each time.

Innocent Blood

Firebrand Archer

Whenever you cast a noncreature spell, Firebrand Archer deals 1 damage to each opponent.

Electrodominance

Electrodominance deals X damage to any target. You may cast a spell with mana value X or less from your hand without paying its mana cost.

Spiteful Visions

At the beginning of each player's draw step, that player draws an additional card.
Whenever a player draws a card, Spiteful Visions deals 1 damage to that player.

Underworld Dreams

Whenever an opponent draws a card, Underworld Dreams deals 1 damage to that player.

Smallpox

Each player loses 1 life, discards a card, sacrifices a creature, then sacrifices a land.

Torbran, Thane of Red Fell

If a red source you control would deal damage to an opponent or a permanent an opponent controls, it deals that much damage plus 2 instead.

Torment of Hailfire

Repeat the following process X times. Each opponent loses 3 life unless that player sacrifices a nonland permanent or discards a card.

Lightning Reaver

Haste; fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
Whenever Lightning Reaver deals combat damage to a player, put a charge counter on it.
At the beginning of your end step, Lightning Reaver deals damage equal to the number of charge counters on it to each opponent.

Painful Quandary

Whenever an opponent casts a spell, that player loses 5 life unless they discard a card.

Last One Standing

Choose a creature at random, then destroy the rest.

Swamp

Crypt Ghast

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Whenever you tap a Swamp for mana, add an additional {B}.

Fate Unraveler

Whenever an opponent draws a card, Fate Unraveler deals 1 damage to that player.

Ruby Medallion

Red spells you cast cost {1} less to cast.

Syphon Mind

Each other player discards a card. You draw a card for each card discarded this way.

Tree of Perdition

Defender
{T}: Exchange target opponent's life total with Tree of Perdition's toughness.

Triskaidekaphobia

At the beginning of your upkeep, choose one —
• Each player with exactly 13 life loses the game, then each player gains 1 life.
• Each player with exactly 13 life loses the game, then each player loses 1 life.

Smoldering Marsh

({T}: Add {B} or {R}.)
Smoldering Marsh enters tapped unless you control two or more basic lands.

Bedevil

Destroy target artifact, creature, or planeswalker.

Volcanic Fallout

This spell can't be countered.
Volcanic Fallout deals 2 damage to each creature and each player.

Rakdos Guildgate

Rakdos Guildgate enters tapped.
{T}: Add {B} or {R}.

Rain of Gore

If a spell or ability would cause its controller to gain life, that player loses that much life instead.

Arcbond

Choose target creature. Whenever that creature is dealt damage this turn, it deals that much damage to each other creature and each player.

Night's Whisper

Chain Reaction

Chain Reaction deals X damage to each creature, where X is the number of creatures on the battlefield.

Everlasting Torment

Players can't gain life.
Damage can't be prevented.
All damage is dealt as though its source had wither. (A source with wither deals damage to creatures in the form of -1/-1 counters.)

Inferno

Inferno deals 6 damage to each creature and each player.

Dictate of the Twin Gods

Flash
If a source would deal damage to a permanent or player, it deals double that damage to that permanent or player instead.

Mountain

Shadowblood Ridge

War's Toll

Whenever an opponent taps a land for mana, tap all lands that player controls.
If a creature an opponent controls attacks, all creatures that opponent controls attack if able.

Killing Wave

For each creature, its controller sacrifices it unless they pay X life.

Sulfuric Vortex

At the beginning of each player's upkeep, Sulfuric Vortex deals 2 damage to that player.
If a player would gain life, that player gains no life instead.

Decree of Annihilation

Exile all artifacts, creatures, and lands from the battlefield, all cards from all graveyards, and all cards from all hands.
Cycling {5}{R}{R} ({5}{R}{R}, Discard this card: Draw a card.)
When you cycle Decree of Annihilation, destroy all lands.

Rampaging Ferocidon

Menace
Players can't gain life.
Whenever another creature enters, Rampaging Ferocidon deals 1 damage to that creature's controller.

Smoke

Players can't untap more than one creature during their untap steps.

Death Pits of Rath

Whenever a creature is dealt damage, destroy it. It can't be regenerated.

Temple of Malice

Temple of Malice enters tapped.
When Temple of Malice enters, scry 1.
{T}: Add {B} or {R}.