The Mythic Underworld and Mythic Otherworld

The working title for the new campaign setting is New Troy. This is purely because I've always liked the completely made-up notion from the Middle Ages that Britain was founded by refugees from the fall of Troy. This is a reality in which that, or something like it, happened. But it was long ago, and nobody really knows much about it at all - or what Troy is or was.

In New Troy, there are two other worlds, called Faerie and Muspel. (There is a third, called Elysium, but it remains inaccessible except to the dead.) Faerie is the realm of elves, changelings, and other beings that are by turns capricious, curious, benevolent or cruel. There, time moves more quickly than it does in New Troy, and nothing is ever quite what it seems.

Muspel is the underworld, the realm of evil spirits, giants, and devils. Sometimes it is like fire; other times like ice. There, time moves more slowly than it does in New Troy, and everything is what it seems: malevolent and depraved.

There are times, and places, where the boundaries between New Troy and the world of Faerie and the world of Muspel become blurred. For Faerie, these tend to be ever-shifting: a gate may exist on a certain hilltop for a hundred years before suddenly moving elsewhere or closing, while another may be found behind a waterfall for one day every decade. For Muspel, they tend to be fixed, eternal: caves, chasms, natural arches.

Faerie and Muspel are where adventure happens in New Troy. Muspel is the mythic underworld as described by Philotomy:

There is a school of thought on dungeons that says they should have been built with a distinct purpose, should "make sense" as far as the inhabitants and their ecology, and shouldn't necessarily be the centerpiece of the game (after all, the Mines of Moria were just a place to get through). None of that need be true for a megadungeon underworld. There might be a reason the dungeon exists, but there might not; it might simply be. It certainly can, and perhaps should, be the centerpiece of the game. As for ecology, a megadungeon should have a certain amount of verisimilitude and internal consistency, but it is an underworld: a place
where the normal laws of reality may not apply, and may be bent, warped, or broken. Not merely an underground site or a lair, not sane, the underworld gnaws on the physical world like some chaotic cancer.  

It is inimical to men; the dungeon, itself, opposes and obstructs the adventurers brave enough to explore it.

Faerie is the mythic otherworld, the wilderness equivalent to the megadungeon. A world of trees, hills, rivers and rain, but where the normal laws of reality do not apply, and are bent, warped or broken. It may even be more dangerous than Muspel, because it resembles the real world so closely that you may not even realise you are in it. But it is not the real world: There are other things there.