Use the
Vermin Nest to summon
Nibbles the Putrid Rat (level 30) in your castle to unlock Rat Form. Next, travel to the
Sacred Silver Mine in Silverlight and enter the mine through the passage along the east side of the mine. Once inside, look for the pack of rats near the north wall. Then, transform into a rat and interact with the hole to pass through the small wall into a hidden room with chests.