The Underworld

 is found within the Main Hall of the Temple, accessed by interacting with the stone image on the floor at the center of the room. It is used to select Blessings, purchase permanent upgrades and unlock weapons.

Blessings
Blessings are selected and unlocked from the blessings tab of the upgrade menu accessed at the center of the Underworld. Each blessing costs Crystal Skulls to unlock, and only up to 3 may be selected at a time.

Forsaken Weaponry
The Forsaken Weaponry tab of the Underworld is used to unlock new weapons using Jade Rings.

Additionally, weapons can then be banished after they’ve been unlocked, preventing them from appearing in explorations again until they are unbanished. Neither banishing nor unbanishing comes at any cost to the player. However, weapons which were starting unlocks can't be banished, and are always permanent.

Weapon Altars
The Weapon Altars tab of the Underworld is used to increase the chances for better items at weapon altars in the Main Hall, as well as repair broken altars. All purchases here are done using Crystal Skulls.

Divine Favors
Divine Favors are purchased within the Underworld using Crystal Skulls, with each one costing an increasingly higher amount with the more that are bought, with a maximum of 10.