When you reach the 15 minute mark on Moongolow, you will be transported to a new stage where you will lose all weapons/items. Then, you will need to run to the end of the area while avoiding enemies to find the Yellow Sign off the edge of the level. Once you have the sign, you will unlock Silver/Gold Rings and Metaglio Left/Right, and enables hidden relics (Great Gospel and Forbidden Scrolls) and events in the other stages.