Dead Weight

Enchant creature
Enchanted creature gets -2/-2.

Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

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.

Marauding Blight-Priest

Whenever you gain life, each opponent loses 1 life.

Sign in Blood

Target player draws two cards and loses 2 life.

Child of Night

Crippling Blight

Enchant creature
Enchanted creature gets -1/-1 and can't block.

Phyrexian Rager

When Phyrexian Rager enters, you draw a card and you lose 1 life.

Supernatural Stamina

Until end of turn, target creature gets +2/+0 and gains "When this creature dies, return it to the battlefield tapped under its owner's control."

Typhoid Rats

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)

Bone Splinters

As an additional cost to cast this spell, sacrifice a creature.
Destroy target creature.

Swamp

Witch's Cottage

({T}: Add {B}.)
Witch's Cottage enters tapped unless you control three or more other Swamps.
When Witch's Cottage enters untapped, you may put target creature card from your graveyard on top of your library.

Vermin Gorger

{T}, Sacrifice another creature: Each opponent loses 2 life and you gain 2 life.

Echoing Return

Return target creature card and all other cards with the same name as that card from your graveyard to your hand.

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.