Marrow-Gnawer

All Rats have fear.
{T}, Sacrifice a Rat: Create X 1/1 black Rat creature tokens, where X is the number of Rats you control.

Crypt Rats

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

Pack Rat's power and toughness are each equal to the number of Rats you control.
{2}{B}, Discard a card: Create a token that's a copy of Pack Rat.

At the beginning of each opponent's upkeep, that player loses 1 life for each card fewer than three in their hand.

{1}{B}, {T}: Target opponent discards a card. Then if that player has no cards in hand, flip Nezumi Shortfang.

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

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