Neutralize the Guards

Creatures target opponent controls get -1/-1 until end of turn. Surveil 2. (Look at the top two cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)

Fell

Odylic Wraith

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
Whenever this creature deals damage to a player, that player discards a card.

Blistergrub

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
When this creature dies, each opponent loses 2 life.

Festering March

Creatures your opponents control get -1/-1 until end of turn. Exile Festering March with three time counters on it.
Suspend 3—{2}{B} (Rather than cast this card from your hand, you may pay {2}{B} and exile it with three time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost.)

The Black Breath

Creatures your opponents control get -1/-1 until end of turn. The Ring tempts you.

Liliana of the Dark Realms

+1: Search your library for a Swamp card, reveal it, put it into your hand, then shuffle.
−3: Target creature gets +X/+X or -X/-X until end of turn, where X is the number of Swamps you control.
−6: You get an emblem with "Swamps you control have '{T}: Add {B}{B}{B}{B}.'"

Caustic Crawler

Landfall — Whenever a land you control enters, you may have target creature get -1/-1 until end of turn.

Ascendant Evincar

Flying (This creature can't be blocked except by creatures with flying or reach.)
Other black creatures get +1/+1.
Nonblack creatures get -1/-1.

Plague Mare

This creature can't be blocked by white creatures.
When this creature enters, creatures your opponents control get -1/-1 until end of turn.

Cower in Fear

Creatures your opponents control get -1/-1 until end of turn.

Thought Vessel

You have no maximum hand size.
{T}: Add {C}.

Iridescent Vinelasher

Offspring {2} (You may pay an additional {2} as you cast this spell. If you do, when this creature enters, create a 1/1 token copy of it.)
Landfall — Whenever a land you control enters, this creature deals 1 damage to target opponent.

Saw in Half

Destroy target creature. If that creature dies this way, its controller creates two tokens that are copies of that creature, except their power is half that creature's power and their toughness is half that creature's toughness. Round up each time.

Feed the Cycle

As an additional cost to cast this spell, forage or pay {B}. (To forage, exile three cards from your graveyard or sacrifice a Food.)
Destroy target creature or planeswalker.

Outbreak

You may discard a Swamp card rather than pay this spell's mana cost.
Choose a creature type. All creatures of that type get -1/-1 until end of turn.

Doomwake Giant

Constellation — Whenever this creature or another enchantment you control enters, creatures your opponents control get -1/-1 until end of turn.

Moor Fiend

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)

Maha, Its Feathers Night

Flying, trample
Ward—Discard a card.
Creatures your opponents control have base toughness 1.

Plague Beetle

Consumed by Greed

Gift a card (You may promise an opponent a gift as you cast this spell. If you do, they draw a card before its other effects.)
Target opponent sacrifices a creature with the greatest power among creatures they control. If the gift was promised, return target creature card from your graveyard to your hand.

Night Clubber

When this creature enters, creatures your opponents control get -1/-1 until end of turn.
Blitz {2}{B} (If you cast this spell for its blitz cost, it gains haste and "When this creature dies, draw a card." Sacrifice it at the beginning of the next end step.)

Terror

Destroy target nonartifact, nonblack creature. It can't be regenerated.

Gruesome Realization

Choose one —
• You draw two cards and you lose 2 life.
• Creatures your opponents control get -1/-1 until end of turn.

Festergloom

Nonblack creatures get -1/-1 until end of turn.

Sheoldred, Whispering One

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.
At the beginning of each opponent's upkeep, that player sacrifices a creature of their choice.

Suffocating Fumes

Creatures your opponents control get -1/-1 until end of turn.
Cycling {2} ({2}, Discard this card: Draw a card.)

Murder

Swamp

Plague Engineer

Deathtouch
As this creature enters, choose a creature type.
Creatures of the chosen type your opponents control get -1/-1.

Marsh Casualties

Kicker {3} (You may pay an additional {3} as you cast this spell.)
Creatures target player controls get -1/-1 until end of turn. If this spell was kicked, those creatures get -2/-2 until end of turn instead.

Jet Medallion

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

Urborg, Tomb of Yawgmoth

Each land is a Swamp in addition to its other land types.

Filth

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
As long as this card is in your graveyard and you control a Swamp, creatures you control have swampwalk.

Helm of the Host

At the beginning of combat on your turn, create a token that's a copy of equipped creature, except the token isn't legendary. That token gains haste.
Equip {5}

Curse of Death's Hold

Enchant player
Creatures enchanted player controls get -1/-1.

Make Obsolete

Minister of Pain

Exploit (When this creature enters, you may sacrifice a creature.)
When this creature exploits a creature, creatures your opponents control get -1/-1 until end of turn.

Sol Ring

Engineered Plague

As this enchantment enters, choose a creature type.
All creatures of the chosen type get -1/-1.