I had a dream where I was playing Valheim and you went through a portal almost Minecraft style to get to another dimension, except there was literally no light. No moon, no light producing plants, no sun, it was pretty creepy. Your only option for long term light was to plant the guck trees from the swamps. There was also as far as I remember no plants because there was no light to sustain them. So getting things like resin and wood for campfires or torches was pretty hard.
Maybe it could be the underside of the world, since it's basically a flat plane lore wise. It would only be unlocked after you kill the original bosses, then let's say the stone slab you land on opens up into a portal and if you go through it you find yourself in the "underworld" with the actual bosses and monsters who have hid from Zeus's watchful eye. Just a thought.
Edit: Odin's eye sorry I just woke up