Tandem Lookout

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.)
As long as Tandem Lookout is paired with another creature, each of those creatures has "Whenever this creature deals damage to an opponent, draw a card."

Manabarbs

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

Arcane Signet

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

Curiosity

Enchant creature
Whenever enchanted creature deals damage to an opponent, you may draw a card.

Erebor Flamesmith

Whenever you cast an instant or sorcery spell, Erebor Flamesmith deals 1 damage to each opponent.

Repercussion

Whenever a creature is dealt damage, Repercussion deals that much damage to that creature's controller.

Sol Ring

Circle of Flame

Whenever a creature without flying attacks you or a planeswalker you control, Circle of Flame deals 1 damage to that creature.

Grapeshot

Grapeshot deals 1 damage to any target.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)

Saheeli Rai

+1: Scry 1. Saheeli Rai deals 1 damage to each opponent.
−2: Create a token that's a copy of target artifact or creature you control, except it's an artifact in addition to its other types. That token gains haste. Exile it at the beginning of the next end step.
−7: Search your library for up to three artifact cards with different names, put them onto the battlefield, then shuffle.

Raid Bombardment

Whenever a creature you control with power 2 or less attacks, Raid Bombardment deals 1 damage to the player or planeswalker that creature is attacking.

Niv-Mizzet, Parun

This spell can't be countered.
Flying
Whenever you draw a card, Niv-Mizzet, Parun deals 1 damage to any target.
Whenever a player casts an instant or sorcery spell, you draw a card.

Ghyrson Starn, Kelermorph

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
Three Autostubs — Whenever another source you control deals exactly 1 damage to a permanent or player, Ghyrson Starn, Kelermorph deals 2 damage to that permanent or player.

Tectonic Hazard

Tectonic Hazard deals 1 damage to each opponent and each creature they control.

Gelectrode

{T}: Gelectrode deals 1 damage to any target.
Whenever you cast an instant or sorcery spell, you may untap Gelectrode.

Grafted Exoskeleton

Equipped creature gets +2/+2 and has infect. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Whenever Grafted Exoskeleton becomes unattached from a permanent, sacrifice that permanent.
Equip {2}

Exalted Flamer of Tzeentch

Sorcerous Inspiration — At the beginning of your upkeep, return an instant or sorcery card at random from your graveyard to your hand.
Fire of Tzeentch — Whenever you cast an instant or sorcery spell, Exalted Flamer of Tzeentch deals 1 damage to each opponent.

Shadowspear

Equipped creature gets +1/+1 and has trample and lifelink.
{1}: Permanents your opponents control lose hexproof and indestructible until end of turn.
Equip {2}

Ruby Medallion

Red spells you cast cost {1} less to cast.

Aether Sting

Whenever an opponent casts a creature spell, Aether Sting deals 1 damage to that player.

Electrostatic Field

Defender
Whenever you cast an instant or sorcery spell, Electrostatic Field deals 1 damage to each opponent.

Chandra, Awakened Inferno

This spell can't be countered.
+2: Each opponent gets an emblem with "At the beginning of your upkeep, this emblem deals 1 damage to you."
−3: Chandra, Awakened Inferno deals 3 damage to each non-Elemental creature.
−X: Chandra, Awakened Inferno deals X damage to target creature or planeswalker. If a permanent dealt damage this way would die this turn, exile it instead.

Unruly Catapult

Defender
{T}: Unruly Catapult deals 1 damage to each opponent.
Whenever you cast an instant or sorcery spell, untap Unruly Catapult.

Firebrand Archer

Whenever you cast a noncreature spell, this creature deals 1 damage to each opponent.

Basilisk Collar

Equipped creature has deathtouch and lifelink. (Any amount of damage it deals to a creature is enough to destroy it. Damage dealt by this creature also causes you to gain that much life.)
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Invert Polarity

Choose target spell, then flip a coin. If you win the flip, gain control of that spell and you may choose new targets for it. If you lose the flip, counter that spell.

Thermo-Alchemist

Defender
{T}: Thermo-Alchemist deals 1 damage to each opponent.
Whenever you cast an instant or sorcery spell, untap Thermo-Alchemist.

Kessig Flamebreather

Whenever you cast a noncreature spell, Kessig Flamebreather deals 1 damage to each opponent.

Fellwar Stone

{T}: Add one mana of any color that a land an opponent controls could produce.

Rhystic Study

Whenever an opponent casts a spell, you may draw a card unless that player pays {1}.

Scouring Sands

Scouring Sands deals 1 damage to each creature your opponents control. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Blazing Volley

Blazing Volley deals 1 damage to each creature your opponents control.

Gorgon's Head

Equipped creature has deathtouch.
Equip {2}

Impact Tremors

Whenever a creature you control enters, this enchantment deals 1 damage to each opponent.

Burning Earth

Whenever a player taps a nonbasic land for mana, Burning Earth deals 1 damage to that player.

Electrickery

Electrickery deals 1 damage to target creature you don't control.
Overload {1}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Quietus Spike

Equipped creature has deathtouch.
Whenever equipped creature deals combat damage to a player, that player loses half their life, rounded up.
Equip {3}

Ophidian Eye

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Whenever enchanted creature deals damage to an opponent, you may draw a card.

Smash to Dust

Choose one —
• Destroy target artifact.
• Destroy target creature with defender.
• Smash to Dust deals 1 damage to each creature your opponents control.

Caltrops

Whenever a creature attacks, Caltrops deals 1 damage to it.

Goblin Bombardment

Sacrifice a creature: Goblin Bombardment deals 1 damage to any target.

End the Festivities

End the Festivities deals 1 damage to each opponent and each creature and planeswalker they control.

Talisman of Creativity

{T}: Add {C}.
{T}: Add {U} or {R}. Talisman of Creativity deals 1 damage to you.

Ral, Storm Conduit

Whenever you cast or copy an instant or sorcery spell, Ral, Storm Conduit deals 1 damage to target opponent or planeswalker.
+2: Scry 1.
−2: When you next cast an instant or sorcery spell this turn, copy that spell. You may choose new targets for the copy.

Cavalcade of Calamity

Whenever a creature you control with power 1 or less attacks, Cavalcade of Calamity deals 1 damage to the player or planeswalker that creature is attacking.