Urborg, Tomb of Yawgmoth

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

Jasmine Boreal

Kormus Bell

All Swamps are 1/1 black creatures that are still lands.

Roots of Life

As Roots of Life enters the battlefield, choose Island or Swamp.
Whenever a land of the chosen type an opponent controls becomes tapped, you gain 1 life.

Bog-Strider Ash

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
Whenever a player casts a Goblin spell, you may pay {G}. If you do, you gain 2 life.

Marsh Threader

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

Stern Judge

{T}: Each player loses 1 life for each Swamp they control.

Farbog Explorer

Thelon's Chant

At the beginning of your upkeep, sacrifice Thelon's Chant unless you pay {G}.
Whenever a player puts a Swamp onto the battlefield, Thelon's Chant deals 3 damage to that player unless the player puts a -1/-1 counter on a creature they control.

Cho-Arrim Legate

If an opponent controls a Swamp and you control a Plains, you may cast this spell without paying its mana cost.
Protection from black

Nature's Wrath

At the beginning of your upkeep, sacrifice Nature's Wrath unless you pay {G}.
Whenever a player puts an Island or blue permanent onto the battlefield, that player sacrifices an Island or blue permanent.
Whenever a player puts a Swamp or black permanent onto the battlefield, that player sacrifices a Swamp or black permanent.

Spreading Algae

Enchant Swamp
When enchanted land becomes tapped, destroy it.
When Spreading Algae is put into a graveyard from the battlefield, return Spreading Algae to its owner's hand.

Marsh Boa

Anaconda

Mire Boa

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
{G}: Regenerate Mire Boa. (The next time this creature would be destroyed, instead tap it, remove it from combat, and heal all damage on it.)

Angry Mob

Trample
As long as it's your turn, Angry Mob's power and toughness are each equal to 2 plus the number of Swamps your opponents control. As long as it's not your turn, Angry Mob's power and toughness are each 2.

Crusading Knight

Protection from black
Crusading Knight gets +1/+1 for each Swamp your opponents control.

Warthog

Drought

At the beginning of your upkeep, sacrifice Drought unless you pay {W}{W}.
Spells cost an additional "Sacrifice a Swamp" to cast for each black mana symbol in their mana costs.
Activated abilities cost an additional "Sacrifice a Swamp" to activate for each black mana symbol in their activation costs.

Pygmy Allosaurus

Royal Decree

Cumulative upkeep {W} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Whenever a Swamp, Mountain, black permanent, or red permanent becomes tapped, Royal Decree deals 1 damage to that permanent's controller.

Yavimaya, Cradle of Growth

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

Karma

At the beginning of each player's upkeep, Karma deals damage to that player equal to the number of Swamps they control.