Underworld

The Underworld is located in a passageway deep in the middle of a canyon. It's a dangerous and long trek to find the entrance, and once there, Cerberus's lava blocks the only way to get in. From the entrance, one must travel through the tunnel a mile before it opens into a massive cave. It is extremely hot inside the cave because Cerberus keeps it that way. Mortals who manage to gain access shouldn't stay long.
Gods and goddesses only.
Souls only.
Mortals must be granted access by Cerberus.