Phaeocollybia arduennensis is a deep-rooting agaric fungus in the family Hymenogastraceae, assessed as Endangered (EN). It grows in old-growth conifer and mixed forests, producing distinctive tawny-brown, conical caps and a tapering pseudorhiza anchored deep in the soil. Its endangered status reflects dependence on undisturbed, mature forest with deep, humus-rich soils.