Manabarbs

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

Fire Diamond

This artifact enters tapped.
{T}: Add {R}.

Spellshock

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

Sol Ring

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.

Harsh Mentor

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

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.