From VG247: “A parallel world running beneath our feet, where unseen denizens plot the demise of the people on the surface.
Its an idea youll find in Warhammer in the form of the Skaven Under-Empire, the armpit realm of tunnels that underlays entire continents like gas pipes. It pops up in mainstream culture too, in Jordan Peeles Us, and even the Best Picture Oscar winning Parasite. Its a shared nightmare that speaks to our unease about class and hierarchy the comeuppance those at the top fear from those below.”
Source: N4G PC Will Baldur's Gate 3 go to the Underdark?