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