Manabarbs

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

Price of Glory

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

Descent into Avernus

At the beginning of your upkeep, put two descent counters on Descent into Avernus. Then each player creates X Treasure tokens and Descent into Avernus deals X damage to each player, where X is the number of descent counters on Descent into Avernus.

Armageddon Clock

At the beginning of your upkeep, put a doom counter on Armageddon Clock.
At the beginning of your draw step, Armageddon Clock deals damage equal to the number of doom counters on it to each player.
{4}: Remove a doom counter from Armageddon Clock. Any player may activate this ability but only during any upkeep step.

Fiery Emancipation

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

Rakdos Charm

Choose one —
• Exile target player's graveyard.
• Destroy target artifact.
• Each creature deals 1 damage to its controller.

Price of Progress

Price of Progress deals damage to each player equal to twice the number of nonbasic lands that player controls.

Ankh of Mishra

Whenever a land enters the battlefield, Ankh of Mishra deals 2 damage to that land's controller.

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.

Fires of Mount Doom

When Fires of Mount Doom enters the battlefield, it deals 2 damage to target creature an opponent controls. Destroy all Equipment attached to that creature.
{2}{R}: Exile the top card of your library. You may play that card this turn. When you play a card this way, Fires of Mount Doom deals 2 damage to each player.

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.

Spawn of Mayhem

Spectacle {1}{B}{B} (You may cast this spell for its spectacle cost rather than its mana cost if an opponent lost life this turn.)
Flying, trample
At the beginning of your upkeep, Spawn of Mayhem deals 1 damage to each player. Then if you have 10 or less life, put a +1/+1 counter on Spawn of Mayhem.

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.

Zo-Zu the Punisher

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

Rampaging Ferocidon

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

Heartless Hidetsugu

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

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.

Sire of Insanity

At the beginning of each end step, each player discards their hand.

Havoc Festival

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

Bellowing Mauler

At the beginning of your end step, each player loses 4 life unless they sacrifice a nontoken creature.

Seizan, Perverter of Truth

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

Roiling Vortex

At the beginning of each player's upkeep, Roiling Vortex deals 1 damage to them.
Whenever a player casts a spell, if no mana was spent to cast that spell, Roiling Vortex deals 5 damage to that player.
{R}: Your opponents can't gain life this turn.

Scytheclaw Raptor

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

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.

Spellshock

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

Dire Fleet Ravager

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

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.

Plague Spitter

At the beginning of your upkeep, Plague Spitter deals 1 damage to each creature and each player.
When Plague Spitter dies, it deals 1 damage to each creature and each player.

Blasphemous Act

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

Rug of Smothering

Flying
Whenever a player casts a spell, they lose 1 life for each spell they've cast this turn.