Chain Reaction

Chain Reaction deals X damage to each creature, where X is the number of creatures on the battlefield.

Boros Reckoner

Whenever Boros Reckoner is dealt damage, it deals that much damage to any target.
{R/W}: Boros Reckoner gains first strike until end of turn.

Brash Taunter

Indestructible
Whenever Brash Taunter is dealt damage, it deals that much damage to target opponent.
{2}{R}, {T}: Brash Taunter fights another target creature.

Spitemare

Whenever Spitemare is dealt damage, it deals that much damage to any target.

Aether Flash

Whenever a creature enters, Aether Flash deals 2 damage to it.

Swords to Plowshares

Exile target creature. Its controller gains life equal to its power.

Tamanoa

Whenever a noncreature source you control deals damage, you gain that much life.

Blasphemous Act

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

Pyrohemia

At the beginning of the end step, if no creatures are on the battlefield, sacrifice Pyrohemia.
{R}: Pyrohemia deals 1 damage to each creature and each player.

Toralf's Hammer

Equipped creature has "{1}{R}, {T}, Unattach Toralf's Hammer: It deals 3 damage to any target. Return Toralf's Hammer to its owner's hand."
Equipped creature gets +3/+0 as long as it's legendary.
Equip {1}{R}

Toralf, God of Fury

Trample
Whenever a creature or planeswalker an opponent controls is dealt excess noncombat damage, Toralf deals damage equal to the excess to any target other than that permanent.

Toralf, God of Fury // Toralf's Hammer

Firesong and Sunspeaker

Red instant and sorcery spells you control have lifelink.
Whenever a white instant or sorcery spell causes you to gain life, Firesong and Sunspeaker deals 3 damage to target creature or player.

Stuffy Doll

Indestructible
As Stuffy Doll enters, choose a player.
Whenever Stuffy Doll is dealt damage, it deals that much damage to the chosen player.
{T}: Stuffy Doll deals 1 damage to itself.