The Secret Room is a special, small location with a hidden entrance and a treasure chest inside.
You can get into the Secret Room only by blowing up its wall with a powder bomb. The bomb slot will only become active once you find the right niche in the wall. The sole sure way to find the Secret Room is by gaining access to the second floor of the Dungeon of the Torturers. You can also find the Secret Room in the Dungeon of the Forsaken, but only during special seasonal Halloween and Christmas updates. However, there is no way to guarantee that the room will appear during seasonal updates.