Trapdoor: Rules for Tabletop Dungeon Crawls (2014)

From the introduction:

Prise open the trapdoor...

...AND descend into the inky depths below in search of fortune and glory! Trapdoor is a set of rules which allow players to explore perilous subterranean fantasy worlds using little more than dice, miniatures and their imaginations. It was born chiefly from two desires: 1. to devise a simple dungeon adventure rules engine that still allowed for detailed character development and thus longevity and 2. to create a set free from the constraints of being tied to a predetermined sword & sorcery background. In this way players can use whatever fantasy miniatures they have to hand and can place their games in whatever 'world' they like. The bestiary chapter (p.27) gives profiles of numerous generic fantasy creatures - from Goblins Warriors to Fire Drakes - but gamers are also free to create their own too.

Trapdoor is also scale-free, so whether you want to adventure using 28mm or 10mm scale miniature or anything in-between the choice is yours. Whatever divergent fantastical lands you wish to explore Trapdoor provides the tools for you to do so!