Secrets are places or things that are hidden in levels that can be found in hidden areas. They are usually Chests or Rooms that are hidden somewhere in the level or the rune room. The chests that are secret are most likely going to be Fancy or Obsidian, meanwhile secret rooms can generate multiple high-tier loot chests inside them. There are also map secrets, which are areas that contain maps that can unlock secret locations. There are a total of 6 secret levels in the Mainland, 5 of which can be obtained finding scrolls hidden in their parent level. The number of secrets in a level is shown on the bottom left corner of the level map, which is accessed by pressing M. This can be useful for determining if the secret you are looking for is in that level or not. Rune locations also count as secrets and can appear in all mainland non-secret levels. They can be redeemed at the Church to unlock a hidden room that holds the map for ??? and 2 obsidian chests. Secret areas can also contain arenas and even exclusive mobs in some cases.