Nightmare is a song that first appeared in Akumajou Densetsu (FC). It played while you were inside Alucard's Cave and it has been remixed in many other Castlevania games as well. It is also considered to be the theme of Alucard in Castlevania III: Dracula's Curse (NES) .
Akumajou Densetsu (FC)
Castlevania III: Dracula's Curse (NES)
Castlevania: Circle of the Moon (GBA)