Complex sentence: Its foundation is the Horus Heresy, a cornerstone event of the dystopian, science fantasy Warhammer 40,000 universe; the shared universe was originally created in 1987 by Black Library parent company Games Workshop as the campaign setting for its related tabletop miniatures wargame.
Split into simpler sentences: Its foundation is the Horus Heresy, a cornerstone event of the dystopian, science fantasy Warhammer 40,000 universe. The shared universe was originally created in 1987 by Games Workshop, parent company of "Series" publisher Black Library, as the campaign setting for the "Warhammer 40,000" tabletop miniatures wargame.