Rat Bond

It charms all rats to fight for the Peasant and attack other enemies in the room; as the rats' attacks have knockback, they can push enemies into pits. The rats cannot be directly attacked and do not need to be defeated to clear the room. Rats can still take damage from other enemies and the environment. The rats will follow the Peasant across rooms and are able to push them onto environmental dangers like spikes and pits.

A charmed boosts other rats' damage and speed.

Blueprint
In order to unlock the blueprint you must help by freeing (and not killing because he will be passive) him from a random box in Goldmines on levels 1-3 after helping him you will find a random crawlspace again in Goldmines on levels 1-3 you will find him alone with a box. Inside the box is Mrs. Rat, destroy the box to get her out.

Trivia

 * This relic is often referred to as "Cheese" due to its icon.