Belly of the Beast has a very unusual premise. An alien giant worm has consumed the surface of a conventional fantasy setting and now the shattered remnants of civilisation live inside the vast intestine of the creature, struggling to scavenge enough to survive from either the creature itself of the remains of its other meals. I guess you might summarise it as post-apocalyptic body horror.
The Swallowed live in small communities learning to live on what they can scavenge within the complex tract of the worm and the body of the creature itself.
Life inside the creature is defined not by day or not or the passing of the seasons but the structure of the creatures body, its movement and the arrival of new resources in the form of newly devoured territory. It really is new levels of "grimdark".
The game is structured around "pulls" which is where the characters try and acquire what their community needs to get through another day. The communities are world-built or can simply be randomly generated (along with their current needs).
The GM creates an opportunity to satisfy that need and the characters go after that opportunity as best they can. It predates Blades in the Dark but there is a lot of overlap in the basic ideas of the adventure play being driven and feeding back into the community cycle. Mechanically though only the adventure play has rules and mechanics. The communities are more abstract.
On the mechanics, during conflicts the GM sets the difficulty of any task or outcome and then the player builds a dice pool based on their character and pools of Advantage and Instinct dice. Advantage dice seem to be GM-awarded fiat that players can request. Instinct dice are more interesting as they are built up by the characters acting out the character's instincts and those instincts are those of desperate survivors. Essentially the Instinct dice enforce the cultural values of the setting. Act according to them and you get rewarded with dice that you can spent to be more successful.
Although the rules explains the mechanics clearly (with handy recaps and summaries at the back of the book) it is a little hard to really understand how the dice flow works without trying it in play.
Belly of the Beast is a highly distinctive setting with a custom ruleset that aims to support player behaviour that matches the setting. Its biggest failings, as written, is putting a burden of both story creation and situational judgement on the GM. It feels like there could have been more support for collaborative situation or world-building and that the mechanics of the challenge could have been based around the structure of the pull.
This is a game I'm curious to play but it does fall between traditional and storytelling stools so it might be difficult to pitch it to an interested group.