Ichor Explosion

As an additional cost to cast this spell, sacrifice a creature.
All creatures get -X/-X until end of turn, where X is the sacrificed creature's power.

Massacre Girl, Known Killer

Menace
Creatures you control have wither. (They deal damage to creatures in the form of -1/-1 counters.)
Whenever a creature an opponent controls dies, if its toughness was less than 1, draw a card.

Terror Tide

Fathomless descent — All creatures get -X/-X until end of turn, where X is the number of permanent cards in your graveyard.

Black Sun's Zenith

Put X -1/-1 counters on each creature. Shuffle Black Sun's Zenith into its owner's library.

Cloudkill

All creatures get -X/-X until end of turn, where X is the greatest mana value of a commander you own on the battlefield or in the command zone.

Toxic Deluge

As an additional cost to cast this spell, pay X life.
All creatures get -X/-X until end of turn.

Final Act

Choose one or more —
• Destroy all creatures.
• Destroy all planeswalkers.
• Destroy all battles.
• Exile all graveyards.
• Each opponent loses all counters.