I think it's bad history. The Western Roman Empire was Catholic for about 500 years before it fell (the ERE Orthodox), and the "fall" was a slow dissolution of the central Imperial authority and its replacement by the existing systems of patronage evolving into fuedal contracts, not some cataclysmic event.

Cults, including what could be called "death" cults, were a constant throughout Roman society, not a sudden appearance that hailed the end times. They might have been more common in and around certain cataclysmic events, like plagues and invasions, as a societal coping mechanism, but that's just speculation.