Oppressive Atmosphere is a mythos card. It first appeared in the Innsmouth Horror expansion.
Mythos Type: Environment (Mystic)
Gate Opens: Devil Reef
Clue Appears At: *Esoteric Order of Dagon
Mythos Ability: While this card is in play, all monsters gain Nightmarish 1. Any monster that already has Nightmarish has its Nightmarish rating increased by 1.