The setting is system-agnostic by design, with four recommended rulesets suited to different tiers and campaign styles. The GM should select based on the intended depth of engagement with horror mechanics and procedural investigation.
Recommended for: full-campaign play, Tier One through Tier Three.
Delta Green is the strongest mechanical fit for the setting as described. Its handling of operational tradecraft, institutional horror, and the psychological cost of continued exposure to the unnatural maps directly onto the investigators' situation. The 'Program vs. the Outlaws' factional structure translates cleanly into Ordus-sanctioned vs. unsanctioned operation. Sanity mechanics are appropriate from mid-Tier One onward. Bonds and their degradation are thematically central.
Adaptation notes: Replace the US government agency framing with Nigel Pennington's network. The Ordus functions as the equivalent of the Program's senior control structure — resources, protocols, and occasional interference.
Recommended for: single-session and short-arc play, Tier One and Two.
CoC's investigative chassis and its Sanity system are genre-appropriate, and its extensive published supplement library provides usable location and monster content. It is somewhat less suited to sustained operational horror than Delta Green but offers lower barrier to entry for players unfamiliar with the genre.
Recommended for: campaigns incorporating dimensional recursion and temporal anomaly play.
The Strange's core mechanic of recursion — stable worlds of belief and narrative that experienced practitioners can travel between — provides an elegant mechanical framework for the setting's time-fold sites, overlapping realities, and the deeper cosmological strata. The Casa Grande temporal incident is a natural Strange recursion. The Celtic temple in the Pacific Northwest suggests a persistent recursion anchored to the site. This system is best deployed in campaigns that are leaning into Tier Three cosmology.
Recommended for: tone calibration and content borrowing.
The Magnus Archives and Old Gods of Appalachia are audio drama sources and ruleset sources, but both provide invaluable tonal and cosmological reference. The Magnus Archives' taxonomy of Fears — and its revelation that those fears are not metaphors but entities — maps onto the setting's Tier Three layer. Old Gods of Appalachia's treatment of ancient presences as genuinely vast and genuinely indifferent, embedded in landscape and community, informs how the setting's deep history should feel at the table.
For mechanics, the Cypher System's abstract harm model (Might/Speed/Intellect pool damage) handles both physical and psychological attrition without the granularity overhead of BRP-derived systems. Suitable for groups who want the tone without the bookkeeping.
System Best For Tier Fit Complexity
Delta Green Full campaigns All tiers Medium-High
Call of Cthulhu 7e Short arcs, one-shots Tier 1–2 Medium
The Strange Dimensional / temporal play Tier 2–3 Medium
Cypher System Tone-first groups Any Low-Medium