This is the entry I'm submitting for this year's 200 Word RPG Challenge, where you need to design a complete tabletop roleplaying game using 200 words or less. This is an unplaytested contest draft.

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No Devil-child May Rule Us

King Obarru is dead. His surly and callow son Darian now rules. Darian's precocious and sophisticated younger sister Hylda gathers a cabal to tell a shocking tale: Queen Alykta consorted with the demon Tchernobog; the man on the throne is a devil-child and not the trueborn heir of Obarru. Hylda needs help developing proof to dispute the succession.

Players: one GM, one Devil, others are Agents.

Agents: Roll 1d6 for Spirit, subtract Spirit from 6 for Body. Choose a name and position in kingdom.

GM: You describe the world and role-play NPCs.

Devil: Secretly roll 1d6 to determine your children: [1: neither, 2-3: Darian, 4-5: Hylda, 6: both]. You don't directly play, use your ability deceive (or reveal truth!) to manipulate things to your liking.

The devil is in the details: Whenever an Agent inspects, probes, or discerns for details roll 1d6. If greater than or equal to Spirit: Devil whispers or passes note to GM telling them what the Agent perceives. The GM describes things as truthfully as they can, consistent with Devil's directives.

To perform difficult or NPC-opposed physical action roll 1d6: under Body succeeds, otherwise you fail, the GM describes how.