Time Terror - Mander

Isn’t that basically the plot of a season in the adventure zone?
There’s a Cypher System RPG called Old Gods of Appalachia that’s pretty neat too.
Old Gods of Appalachia Roleplaying Game - Monte Cook Games Store

Venture into the dark, eerie past of an alternate Appalachia and enjoy a fluid, story-based roleplaying experience with an intuitive, concept-driven character generation system that gives you limitless options. With loads of lore, dozens of creatures and NPCs, and two complete adventures to launch your campaign, the Old Gods of Appalachia Roleplaying Game has everything you need to experience your own stories of horror, hardship, and heart in this immersive, narrative, and deeply chilling setting.

Monte Cook Games Store
Also thematically related is The Twisted Ones by T Kingfisher, which itself is a reinterpretation of The White Ones by Arthur Machen (written in the late 1890s). Appalachia has been creeping people out for a long time!
I loved that novel.