Kaervek the Merciless

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

Mountain

Prisoner's Dilemma

Each opponent secretly chooses silence or snitch, then the choices are revealed. If each opponent chose silence, Prisoner's Dilemma deals 4 damage to each of them. If each opponent chose snitch, Prisoner's Dilemma deals 8 damage to each of them. Otherwise, Prisoner's Dilemma deals 12 damage to each opponent who chose silence.
Flashback {5}{R}{R}

Cruel Entertainment

Choose target player and another target player. The first player controls the second player during the second player's next turn, and the second player controls the first player during the first player's next turn.

Dragonskull Summit

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

Mana Flare

Whenever a player taps a land for mana, that player adds one mana of any type that land produced.

Seizan, Perverter of Truth

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

Game of Chaos

Flip a coin. If you win the flip, you gain 1 life and target opponent loses 1 life, and you decide whether to flip again. If you lose the flip, you lose 1 life and that opponent gains 1 life, and that player decides whether to flip again. Double the life stakes with each flip.

Mayhem Devil

Whenever a player sacrifices a permanent, this creature deals 1 damage to any target.

Sulfuric Vortex

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

Wheel of Misfortune

Each player secretly chooses a number 0 or greater, then all players reveal those numbers simultaneously and determine the highest and lowest numbers revealed this way. Wheel of Misfortune deals damage equal to the highest number to each player who chose that number. Each player who didn't choose the lowest number discards their hand, then draws seven cards.

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.)

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.

Rakdos, Patron of Chaos

Flying, trample
At the beginning of your end step, target opponent may sacrifice two nonland, nontoken permanents of their choice. If they don't, you draw two cards.

Deadpool, Trading Card

As Deadpool enters, you may exchange his text box and another creature's.
At the beginning of your upkeep, you lose 3 life.
{3}, Sacrifice this creature: Each other player draws a card.

Heartless Hidetsugu

{T}: Heartless Hidetsugu deals damage to each player equal to half that player's life total, rounded down.

Harmless Offering

Target opponent gains control of target permanent you control.

Magus of the Wheel

{1}{R}, {T}, Sacrifice this creature: Each player discards their hand, then draws seven cards.

Scytheclaw Raptor

Whenever a player casts a spell, if it's not their turn, this creature deals 4 damage to them.

Deadly Rollick

If you control a commander, you may cast this spell without paying its mana cost.
Exile target creature.

Havoc Festival

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

Head Games

Target opponent puts the cards from their hand on top of their library. Search that player's library for that many cards. The player puts those cards into their hand, then shuffles.

Solphim, Mayhem Dominus

If a source you control would deal noncombat damage to an opponent or a permanent an opponent controls, it deals double that damage to that player or permanent instead.
{1}{R/P}{R/P}, Discard two cards: Put an indestructible counter on Solphim. ({R/P} can be paid with either {R} or 2 life.)

Lavaclaw Reaches

This land enters tapped.
{T}: Add {B} or {R}.
{1}{B}{R}: Until end of turn, this land becomes a 2/2 black and red Elemental creature with "{X}: This creature gets +X/+0 until end of turn." It's still a land.

Command Tower

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

Blasphemous Act

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

Manabarbs

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

Alesha, Who Laughs at Fate

First strike
Whenever Alesha attacks, put a +1/+1 counter on it.
Raid — At the beginning of your end step, if you attacked this turn, return target creature card with mana value less than or equal to Alesha's power from your graveyard to the battlefield.

Chaos Warp

The owner of target permanent shuffles it into their library, then reveals the top card of their library. If it's a permanent card, they put it onto the battlefield.

Swamp

Faithless Looting

Draw two cards, then discard two cards.
Flashback {2}{R} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Rocky Tar Pit

This land enters tapped.
{T}, Sacrifice this land: Search your library for a Swamp or Mountain card, put it onto the battlefield, then shuffle.

Bloodletter of Aclazotz

Flying
If an opponent would lose life during your turn, they lose twice that much life instead. (Damage causes loss of life.)

Goblin Game

Each player hides at least one item, then all players reveal them simultaneously. Each player loses life equal to the number of items they revealed. The player who revealed the fewest items then loses half their life, rounded up. If two or more players are tied for fewest, each loses half their life, rounded up.

Citadel of Pain

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

Grievous Wound

Enchant player
Enchanted player can't gain life.
Whenever enchanted player is dealt damage, they lose half their life, rounded up.

Vandalblast

Destroy target artifact you don't control.
Overload {4}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

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 of their choice.

Canyon Slough

({T}: Add {B} or {R}.)
This land enters tapped.
Cycling {2} ({2}, Discard this card: Draw a card.)

Geothermal Bog

({T}: Add {B} or {R}.)
This land enters tapped.

Infernal Grasp

Destroy target creature. You lose 2 life.

Restless Vents

This land enters tapped.
{T}: Add {B} or {R}.
{1}{B}{R}: Until end of turn, this land becomes a 2/3 black and red Insect creature with menace. It's still a land.
Whenever this land attacks, you may discard a card. If you do, draw a card.

Diabolic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Tainted Peak

{T}: Add {C}.
{T}: Add {B} or {R}. Activate only if you control a Swamp.

Saw in Half

Destroy target creature. If that creature dies this way, its controller creates two tokens that are copies of that creature, except their power is half that creature's power and their toughness is half that creature's toughness. Round up each time.

Sadistic Shell Game

Starting with the next opponent in turn order, each player chooses a creature you don't control. Destroy the chosen creatures.

Foreboding Ruins

As this land enters, you may reveal a Swamp or Mountain card from your hand. If you don't, this land enters tapped.
{T}: Add {B} or {R}.

Xantcha, Sleeper Agent

Xantcha enters under the control of an opponent of your choice.
Xantcha attacks each combat if able and can't attack its owner or planeswalkers its owner controls.
{3}: Xantcha's controller loses 2 life and you draw a card. Any player may activate this ability.

Crackle with Power

Crackle with Power deals five times X damage to each of up to X targets.

Jaya's Immolating Inferno

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Jaya's Immolating Inferno deals X damage to each of up to three targets.

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.