This review is part of a series on thematic OSR games (as defined here). Also note that some of the links below are affiliate links (meaning I get a small percentage of the sale without extra cost to you).
The Nightmares Underneath is a monstrous game sewn together from various bits of both classic and modern games; it combines both old-school and modern design sensibilities in an exemplary way. The basics are simple, but it seems quite complex at first as it addresses a plethora of situations that may arise in play.
The premise of the game is that the mortal realms (collectively called the Kingdom of Dreams, where law and science provide the ruling ideology, as opposed to most fantasy setting's pagan idolatry) are threatened by incursions from the realm of nightmares. The upper levels of these dungeons resemble the mundane world to a certain degree, but the deeper one goes, the more surreal and alien (dare I say nightmarish) they become. Most people are helpless against the influence of these nightmare dungeons and their denizens', except for the player characters.