Havoc Festival

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

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.

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.

Underworld Dreams

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

Zo-Zu the Punisher

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

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.

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, Mayhem Dominus. ({R/P} can be paid with either {R} or 2 life.)

Spellshock

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

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, Manabarbs deals 1 damage to that player.

Painful Quandary

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

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.

Rakdos Charm

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