Defile

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

Sewer Rats

{B}, Pay 1 life: Sewer Rats gets +1/+0 until end of turn. Activate no more than three times each turn.

{X}: Crypt Rats deals X damage to each creature and each player. Spend only black mana on X.

Rat Colony gets +1/+0 for each other Rat you control.
A deck can have any number of cards named Rat Colony.

When Burglar Rat enters, each opponent discards a card.

When Ravenous Rats enters, target opponent discards a card.

When Chittering Rats enters, target opponent puts a card from their hand on top of their library.

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

Target player sacrifices a creature.
Flashback {5}{B}{B} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Plague Rats's power and toughness are each equal to the number of creatures named Plague Rats on the battlefield.

Target player reveals their hand. You choose an artifact or creature card from it. That player discards that card.