Wreckage from the Phyrexian Invasion provided the rats with a seemingly unlimited number of breeding grounds.

Rat Colony

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

Mystery Booster 2

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

For every five you see, there are fifty you don't.

Special Guests

The List

No matter how crowded the den, everyone's welcome to squeak in for the festivities.

Secret Lair Drop

Dominaria

Rat Colony's last ability lets you ignore only the "four-of" rule. It doesn't let you ignore format legality. For example, during a Bloomburrow Limited event, you can't add Rat Colonies from your personal collection.