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

Truefire Captain

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever this creature is dealt damage, it deals that much damage to target player.

Donna Noble

Soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.)
Whenever Donna Noble or a creature it's paired with is dealt damage, Donna Noble deals that much damage to target opponent.
Doctor's companion (You can have two commanders if the other is the Doctor.)

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Spitemare

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

Fiendlash

Equipped creature gets +2/+0 and has reach.
Whenever equipped creature is dealt damage, it deals damage equal to its power to target player or planeswalker.
Equip {2}{R}

Talisman of Hierarchy

{T}: Add {C}.
{T}: Add {W} or {B}. This artifact deals 1 damage to you.

Sol Ring

Star of Extinction

Destroy target land. Star of Extinction deals 20 damage to each creature and each planeswalker.

Talisman of Indulgence

{T}: Add {C}.
{T}: Add {B} or {R}. This artifact deals 1 damage to you.

Boros Reckoner

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

Piru, the Volatile

Flying, lifelink
At the beginning of your upkeep, sacrifice Piru unless you pay {R}{W}{B}.
When Piru dies, it deals 7 damage to each nonlegendary creature.

Blasphemous Act

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

Blazing Sunsteel

Equipped creature gets +1/+0 for each opponent you have.
Whenever equipped creature is dealt damage, it deals that much damage to any target.
Equip {4}

Stuffy Doll

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

Spiteful Sliver

Sliver creatures you control have "Whenever this creature is dealt damage, it deals that much damage to target player or planeswalker."

Mogg Maniac

Whenever this creature is dealt damage, it deals that much damage to target opponent or planeswalker.

Sanguine Bond

Whenever you gain life, target opponent loses that much life.

Barbed Servitor

Indestructible
When this creature enters, suspect it. (It has menace and can't block.)
Whenever this creature deals combat damage to a player, you draw a card and you lose 1 life.
Whenever this creature is dealt damage, target opponent loses that much life.

Talisman of Conviction

{T}: Add {C}.
{T}: Add {R} or {W}. This artifact deals 1 damage to you.

Boros Signet

Arcbond

Choose target creature. Whenever that creature is dealt damage this turn, it deals that much damage to each other creature and each player.

Phyrexian Vindicator

Flying
If damage would be dealt to this creature, prevent that damage. When damage is prevented this way, this creature deals that much damage to any other target.

Vizkopa Guildmage

{1}{W}{B}: Target creature gains lifelink until end of turn.
{1}{W}{B}: Whenever you gain life this turn, each opponent loses that much life.

Pain for All

Enchant creature you control
When this Aura enters, enchanted creature deals damage equal to its power to any other target.
Whenever enchanted creature is dealt damage, it deals that much damage to each opponent.

Pariah's Shield

All damage that would be dealt to you is dealt to equipped creature instead.
Equip {3}

Requiem Monolith

{T}: Until end of turn, target creature gains "Whenever this creature is dealt damage, you draw that many cards and lose that much life." That creature's controller may have this artifact deal 1 damage to it. Activate only as a sorcery.

Coalhauler Swine

Whenever this creature is dealt damage, it deals that much damage to each player.

Rakdos Signet

Orzhov Signet

Brash Taunter

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