“It is fitting that all men should be grateful to the writers of universal history.”
Bibliotheca Historica 1.1
Diodorus spent thirty years assembling a universal history in forty books, running from the creation to Caesar's Gallic wars. About a third survives, much of it myth and ethnography rather than politics.
He is a vital source for Egyptian and Near-Eastern religion, for the deeds of Heracles and Dionysus, and for the rationalising tradition that turned gods into ancient kings — the approach later called euhemerism.
Legacy
His compilations preserve his sources, including writers whose own works are otherwise lost.