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.
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