The Rat Warrens

An "introductory" level to the megadungeon that is the Mountain. I'm not going to pretend it's another Tomb of the Serpent Kings, but I think it does an okay job of teaching some basic dungeoncrawling lessons - carry lots of light, talk to everything, combat has consequences, it's okay to run from fights, et cetera.

It's meant to be played with the GROG and The Mountain at the End of the World Player's Handbook, but (like any OSR-y D&D-y thing) it can be adjusted with fairly little effort to pretty much any other system that you're familiar with. Of note I guess is that it doesn't include morale stats (I just use a Charisma roll) so maybe have morale be 7 +/- CHA mod (which gives ratmen 5 morale and feels appropriate) or assign it by fiat or something.

Adjusting it for a different setting is also pretty simple:
hobgoblins = humans i guess, maybe elves or dwarves or what have you
ratmen, bat men = goblins, kobolds, or just leave them as-is
elves (rooms 10-15 are ancient elf tombs but the assumption is none of the PCs know what the fuck is an elf, in case that wasn't clear) = ancient evil snake people or some other sinister long-dead empire

Cover art is public domain.