La Bas Chartreuse

From publisher blurb:

This is neither an adventure nor campaign setting, but a place where Lovecraftian horror happens!

There's a monolith and nearby stairway down into the earth. La Bas Chartreuse contains various scenario seeds with specifics that can be used to flesh things out. You'll also find several random tables, a couple of which can be used in many, many investigative horror and eldritch pulp scenarios.

Ideally, use La Bas Chartreuse with The Outer Presence. It's dark out there... good luck!