Corrupt

Corrupt deals damage to any target equal to the number of Swamps you control. You gain life equal to the damage dealt this way.

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}.'"

Mind Sludge

Target player discards a card for each Swamp you control.

Magus of the Coffers

{2}, {T}: Add {B} for each Swamp you control.

Squelching Leeches

Squelching Leeches's power and toughness are each equal to the number of Swamps you control.

Griselbrand

Flying, lifelink
Pay 7 life: Draw seven cards.

Dread Presence

Whenever a Swamp you control enters, choose one —
• You draw a card and you lose 1 life.
• Dread Presence deals 2 damage to any target and you gain 2 life.

Defile

Target creature gets -1/-1 until end of turn for each Swamp you control.

Soulhunter Rakshasa

Soulhunter Rakshasa can't block.
When Soulhunter Rakshasa enters, if you cast it from your hand, it deals 1 damage to target opponent for each Swamp you control.

Korlash, Heir to Blackblade

Korlash, Heir to Blackblade's power and toughness are each equal to the number of Swamps you control.
{1}{B}: Regenerate Korlash.
Grandeur — Discard another card named Korlash, Heir to Blackblade: Search your library for up to two Swamp cards, put them onto the battlefield tapped, then shuffle.

Dakmor Sorceress

Dakmor Sorceress's power is equal to the number of Swamps you control.

Nightmare Incursion

Search target player's library for up to X cards, where X is the number of Swamps you control, and exile them. Then that player shuffles.

Sink into Takenuma

Sweep — Return any number of Swamps you control to their owner's hand. Target player discards a card for each Swamp returned this way.

Crypt Ghast

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Whenever you tap a Swamp for mana, add an additional {B}.

Bubbling Muck

Until end of turn, whenever a player taps a Swamp for mana, that player adds an additional {B}.

Quag Sickness

Enchant creature
Enchanted creature gets -1/-1 for each Swamp you control.

Nightmare

Flying (This creature can't be blocked except by creatures with flying or reach.)
Nightmare's power and toughness are each equal to the number of Swamps you control.

Infernal Harvest

As an additional cost to cast this spell, return X Swamps you control to their owner's hand.
Infernal Harvest deals X damage divided as you choose among any number of target creatures.

Nirkana Revenant

Whenever you tap a Swamp for mana, add an additional {B}.
{B}: Nirkana Revenant gets +1/+1 until end of turn.

Tendrils of Corruption

Tendrils of Corruption deals X damage to target creature and you gain X life, where X is the number of Swamps you control.

Mutilate

All creatures get -1/-1 until end of turn for each Swamp you control.

Guul Draz Overseer

Flying
Landfall — Whenever a land you control enters, other creatures you control get +1/+0 until end of turn. If that land is a Swamp, those creatures get +2/+0 until end of turn instead.