In antiquity the priest of Diana at Nemi, near Aricia, was known as the King of the Wood. His rule was limited by a horrific mode of succession. A candidate for this priesthood could succeed to office only by slaying the incumbent priest, and would then retain office until he himself was slain. The murder, however, was constrained by strange rules: within the sacred grove grew a tree of which no branch could be broken save by a runaway slave. If he succeeded in breaking off a branch (which the ancients associated with the Golden Bough that, at the Sibyl's bidding, Aeneas plucked before his journey to Hades), this entitled him to fight the priest in single combat. Success would give him the fearful office of Rex Nemorensis.
Frazer's declared aim in The Golden Bough was to explain this rule of succession, which has no parallel in classical antiquity. He set himself to answer two questions: (1) why had the priest to slay his predecessor? and (2) why, before he slew him, had he to pluck the Golden Bough? (FGB 1890: 6). He held that these questions cannot be answered by reference to classical sources, and accordingly proposed to cast his net worldwide. He wrote:
recent researches into the early history of man have revealed the essential similarity with which, under many superficial differences, the human mind has elaborated its first crude philosophy of life. Accordingly, if we can show that a barbarous custom, like that of the priesthood of Nemi, has existed elsewhere; if we can prove that these motives have operated widely, perhaps universally, in human society, producing in varied circumstances a variety of institutions specifically different but generically alike; if we can show, lastly, that these very motives, with some of their derivative institutions were actually at work in classical antiquity; then we may fairly infer that at a remoter age the same motives gave birth to the priesthood at Nemi. (FGB 1992: 2)