Dungeongating!

A setting premise for the pulp-tinged fantasy or fantasy-tinged pulp RPG of your choice.

While knocking about in the East in the 1920s, an English explorer Sir George Lancasterwood-Smith came into possession of a curious stone ring of impressive size immense age. Naturally he had it shipped back to his estates in Berktonshire, where it stood for several years until one foggy, moonlit night when something came out of the ring and ate Sir George's favorite hound.

 In response, rather than do something sensible (like sticking the ring under an American mountain range for sixty years), Sir George moved the ring indoors and engaged all manner of scientists, cranks, boffins, alchemists, and mystics to figure out its secrets. Eventually, they determined that by illuminating sections of the ring in sequence, the inside would shimmer and thin and, if passed through, would deposit explorers - somewhere else.

Somewhere else, in this case, being an immense underground labyrinth, decorated in a vaguely familiar but unidentifiable style that seems to heavily feature skulls. This was, however, less important than the unique and valuable artifacts that kept turning up, inspiring Sir George to lead several expeditions further and further into the maze.

That, of course, is when the monsters showed up.

The bulk of them seemed to be long-dead bodies mindlessly shambling around despite various states of decomposition, but

there were plenty of other creatures seemingly torn from ghost stories and fairy tales, and others that defied sane descriptions. Despite this, Sir George is still recruiting interested parties to investigate the mysterious underworld, perhaps discovering whether the labyrinth has any exit, or whether three are any other living people inside. Some of those studying the ring - or the Gate, as it's now called - also wonder if, by changing the pattern of the light shone on the ring, the Gate could be made to open somewhere else entirely . . .