Dungeon Delver's Heritages
Many of the species that dwell in Underland’s winding caverns are little known on the surface world. Only the most well-traveled grimlocks, mycelials, oozefolk, rockborn, or motley have seen the light of the sun. Ratlings and doppelgangers, on the other hand, are familiar to, though rarely welcomed by, aboveground communities.
Source