Balefire Dragon

Flying
Whenever Balefire Dragon deals combat damage to a player, it deals that much damage to each creature that player controls.

Chain Reaction

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

Rolling Earthquake

Rolling Earthquake deals X damage to each creature without horsemanship and each player.

Starstorm

Starstorm deals X damage to each creature.
Cycling {3} ({3}, Discard this card: Draw a card.)

Storm's Wrath

Storm's Wrath deals 4 damage to each creature and each planeswalker.

Chandra, Flamecaller

+1: Create two 3/1 red Elemental creature tokens with haste. Exile them at the beginning of the next end step.
0: Discard all the cards in your hand, then draw that many cards plus one.
−X: Chandra, Flamecaller deals X damage to each creature.

Earthquake

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

Hour of Devastation

All creatures lose indestructible until end of turn. Hour of Devastation deals 5 damage to each creature and each non-Bolas planeswalker.

Blasphemous Act

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

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

Star of Extinction

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

Molten Disaster

Kicker {R} (You may pay an additional {R} as you cast this spell.)
If this spell was kicked, it has split second. (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)
Molten Disaster deals X damage to each creature without flying and each player.

Immolating Gyre

Immolating Gyre deals X damage to each creature and planeswalker you don't control, where X is the number of instant and sorcery cards in your graveyard.

Subterranean Tremors

Subterranean Tremors deals X damage to each creature without flying. If X is 4 or more, destroy all artifacts. If X is 8 or more, create an 8/8 red Lizard creature token.