Death Wind

Target creature gets -X/-X until end of turn.

Slice from the Shadows

This spell can't be countered. (This includes by the ward ability.)
Target creature gets -X/-X until end of turn.

Grim Hireling

Whenever one or more creatures you control deal combat damage to a player, create two Treasure tokens.
{B}, Sacrifice X Treasures: Target creature gets -X/-X until end of turn. Activate only as a sorcery.

Priest of the Haunted Edge

{T}, Sacrifice Priest of the Haunted Edge: Target creature gets -X/-X until end of turn, where X is the number of snow lands you control. Activate only as a sorcery.

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.

Contagion Clasp

When Contagion Clasp enters the battlefield, put a -1/-1 counter on target creature.
{4}, {T}: Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Bloodthirsty Ogre

{T}: Put a devotion counter on Bloodthirsty Ogre.
{T}: Target creature gets -X/-X until end of turn, where X is the number of devotion counters on Bloodthirsty Ogre. Activate only if you control a Demon.

Black Sun's Twilight

Up to one target creature gets -X/-X until end of turn. If X is 5 or more, return a creature card with mana value X or less from your graveyard to the battlefield tapped.

Kagemaro, First to Suffer

Kagemaro, First to Suffer's power and toughness are each equal to the number of cards in your hand.
{B}, Sacrifice Kagemaro: All creatures get -X/-X until end of turn, where X is the number of cards in your hand.

Death's Approach

Enchant creature
Enchanted creature gets -X/-X, where X is the number of creature cards in its controller's graveyard.

Battle at the Bridge

Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.)
Target creature gets -X/-X until end of turn. You gain X life.

Essence Pulse

You gain 2 life. Each creature gets -X/-X until end of turn, where X is the amount of life you gained this turn.

Contagion Engine

When Contagion Engine enters the battlefield, put a -1/-1 counter on each creature target player controls.
{4}, {T}: Proliferate twice. (Choose any number of permanents and/or players, then give each another counter of each kind already there. Then do it again.)

Planeswalker's Scorn

{3}{B}: Target opponent reveals a card at random from their hand. Target creature gets -X/-X until end of turn, where X is the revealed card's mana value. Activate only as a sorcery.

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.

Dead of Winter

All nonsnow creatures get -X/-X until end of turn, where X is the number of snow permanents you control.