From the publishers blurb:
Spellbook Games is pleased to announce the release of our next adventure product, Belly of the Beast. Belly of the Beast is a small dungeon location, only 77 rooms, intended for beginning Adventurers. Adventurers have been captured and are dropped into an underground area housing a fearsome beast, their heads covered and their hands bound. A secret ally has hidden equipment in the dungeon, if Adventurers can find it, and there are means to escape. The Adventurers start with nothing so they have to think creatively about both weapons and tactics. This adventure location provides a different way to introduce a group of Adventurers to one another and get them working together.
Belly of the Beast is written using Spellbook Games’ RPG rules, Portal to Adventure. The adventure will require minimal conversion by Games Masters using versions of Dungeons and Dragons from the 1970s and 1980s, or any similar rule set. Games Masters using versions of Dungeons and Dragons from 3.x onward, or any similar rule set, may require extensive conversion.