I see this as a campaign suitable for Call of Cthulhu /Trail of Cthulhu or a similar setting of eldritch horror. I can, however, easily see this as one of: The emperor is a mind sucking alien; or “Solyent Green is people” though some work would need to be done to adapt it (the secret doesn't have any intrinsic power). Alternatively, nano virius, meme or similar device would work in some Sci-Fi settings. For the fantasy setting it would not be out of place to have a Mac Guffin, that if used, would destroy the world. Another good example might be “The Death Note”.
Campaign Plot: A secret that could destroy the world.
Arc Plot 1: A corrupted family is in the process of falling apart. NPCs: A father, a mother, a uncle, and a young adult brother and sister.
Intro Story: A the brother with the assistance of several rakes, assaults the heroes in vengeance for the crime of existing. The confrontation may lead to an apology from the father, or cries of justice from the mother. NPCs: Brother, rakes, father, and the mother.
Story Plot 1: The surviving sibling steals the secret from the family and flees, who gives it to a lover. The family requests the heroes assistance in the recovery of the sibling but makes no mention of the secret. NPCs: Sister, witnesses, minor victims.
Story Plot 2: The lover misuses some of its power and comes into conflict with the authorities who then recover the secret. The heroes are victims of, or witness to the bizarre affects. NPCs: Lover, victims, legal authorities.
Story Plot 3: The secret, now in the hands of a legal authority causes problems at headquarters, and is removed by a clerk who takes it elsewhere.
Interlude Plot 1: A travelogue: as the heroes travel to their next location they are witness to the affects of the secret.
Arc Plot 2: A person attempts to capture the secret.
Story Plot 7: Two factions the new keeper of the secret one to destroy it, the other to use it. During the struggle the secret keeper passes it onto a traveling priest. NPCs: Faction one representatives, Faction two representatives, Secret keeper; Traveling priest.
Story Plot 8: The priest returns to their homeland where they hide it. The secret is, of course, stolen prior to the heroes’ arrival. NPCs: victims of the secret power; Priest; Conflicted clergy and a thief.
Story Plot 9: The thief makes use of the secret’s power and is transformed horribly. The secret in inadvertently acquired by a leader’s assistant.
Interlude Plot 2: Another Travelogue, in which the heroes become witness to the corrupting power of the secret.
Arc Plot 3: From the Assistant the leader gains so much power and influence that they become, effectively, the counties’ political leader.
Story Plot 10: Shows the continuing moral and social decay caused by the secret’s presence in this land. NPCs: The corrupted; the insane; the fearful silent few.
Story Plot 11: The leader has their agents attempt to destroy an ally of the heroes.
Story Plot 12: Investigations by the heroes determines the exact nature of the secret, that the leaders assistant holds it. NPCs: the uncle.
Campaign Climax: The assistant kills the leader and attempts to use the secret final power: the heroes must prevent this. The secret may remain once the ritual is stop. If the ritual is not stopped then horrors will roam the realm again.
Generated by hand from a home brewed tarot generator that used my article “Three Layered Campaign Design”.