Manabarbs

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

Earthquake

Earthquake deals X damage to each creature without flying and each player.

Spellshock

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

Demand Answers

As an additional cost to cast this spell, sacrifice an artifact or discard a card.
Draw two cards.

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.

Auntie Blyte, Bad Influence

Flying
Whenever a source you control deals damage to you, put that many +1/+1 counters on Auntie Blyte, Bad Influence.
{1}{R}, {T}, Remove X +1/+1 counters from Auntie Blyte: It deals X damage to any target.

Scytheclaw Raptor

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

Zo-Zu the Punisher

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

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.

Heartless Hidetsugu

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