Blasphemous Act

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

Shock

Earthquake

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

Lava Dart

Lava Dart deals 1 damage to any target.
Flashback—Sacrifice a Mountain. (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Spikefield Cave

Spikefield Hazard

Spikefield Hazard deals 1 damage to any target. If a permanent dealt damage this way would die this turn, exile it instead.

Spikefield Hazard // Spikefield Cave

Village Rites

As an additional cost to cast this spell, sacrifice a creature.
Draw two cards.

Read the Bones

Scry 2, then draw two cards. You lose 2 life. (To scry 2, look at the top two cards of your library, then put any number of them on the bottom and the rest on top in any order.)

Star of Extinction

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

Volcanic Torrent

Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)
Volcanic Torrent deals X damage to each creature and planeswalker your opponents control, where X is the number of spells you've cast this turn.

Sign in Blood

Target player draws two cards and loses 2 life.

Dark Ritual

Scheming Symmetry

Choose two target players. Each of them searches their library for a card, then shuffles and puts that card on top.

Tectonic Hazard

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

Tainted Strike

Target creature gets +1/+0 and gains infect until end of turn. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

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.")

Judith, Carnage Connoisseur

Whenever you cast an instant or sorcery spell, choose one —
• That spell gains deathtouch and lifelink.
• Create a 2/2 red Imp creature token with "When this token dies, it deals 2 damage to each opponent."

Cast into the Fire

Choose one —
• Cast into the Fire deals 1 damage to each of up to two target creatures.
• Exile target artifact.

Tragic Slip

Target creature gets -1/-1 until end of turn.
Morbid — That creature gets -13/-13 until end of turn instead if a creature died this turn.

Flame Rift

Flame Rift deals 4 damage to each player.

Expedite

Target creature gains haste until end of turn.
Draw a card.

Vandalblast

Destroy target artifact you don't control.
Overload {4}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

End the Festivities

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

Gut Shot

({R/P} can be paid with either {R} or 2 life.)
Gut Shot deals 1 damage to any target.

Chain Reaction

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