Delta Green is a horror TTRPG that fuses the 1920s Cthulhu Mythos with modern conspiracy fiction. It began in 1992 in the fanzine "The Unspeakable Oath," was published in 1997 as a sourcebook for Call of Cthulhu, and was created by Dennis Detwiller, Adam Scott Glancy, and John Tynes.
It became a standalone game in 2016 when Arc Dream Publishing released the "Agent’s Handbook" as its first independent rulebook. It uses a percentile system modified from the BRP / Call of Cthulhu mechanics.
Players take the role of agents in a covert federal network combating unnatural threats. The premise — those who guard the truth are ruined by it — re-dresses cosmic horror in the clothes of espionage and conspiracy.