From Dark and Darker Wiki
After descending into the Inferno, you want to make your way to the Blood Prison. As you stand in front of the iron gate there, you will see the Lions Head or Golden chest in the background. Open the gate and step in. To your left is wall decoration with a matching one to the right. Stand in front of the one on the left, look down at the ground. It's hard to see, it blends well into the floor.