What is Delta Green? - Fall of Delta Green Era

DELTA GREEN is an authorized but unacknowledged black program of the United States national security establishment. Its personnel are tasked from all branches of the federal government, especially from the military, law enforcement, and intelligence communities. The program is classified Top Secret, with its existence and mission known only to holders of the DELTA GREEN clearance.

  The program has a small permanent staff deep in the bowels of the Pentagon, but its agents operate in secret. The program buries DELTA GREEN missions inside other ongoing military operations, or disguises them as training exercises, liaison work, or entirely unrelated investigations. DELTA GREEN agents hold normal federal postings in the military, CIA, FBI, NSA, or some other agency. Their employers never understand the real mission of the program.
  DELTA GREEN began with the raid on Innsmouth, Massachusetts forty years ago, in February 1928. That raid revealed a literally apocalyptic threat to the United States, and to the world. It gave the federal government a glimpse into a parallel physics and biology of alien gods and immortal monsters. Even the briefest contact with these phenomena destroys minds and lives; the touch of the unnatural corrupts and poisons those who study it and those who oppose it. Encounters with this implacably hostile, unutterably incomprehensible universe have leveled civilizations and sunk continents.
  The mission of DELTA GREEN: Hunt and destroy any and all aspects of this unnatural reality at home and abroad. At any cost.