Warhammer 40,000 Codex

A codex (often pluralised as codexes by Games Workshop, though the grammatically correct pluralisation is codices), in the Warhammer 40,000 tabletop wargame, is a rules supplement containing information concerning a particular army, environment, or worldwide campaign. https://en.wikipedia.org/wiki/Codex_(Warhammer_40,000)