What made Nemi's sanctuary famous, both in antiquity and to modern readers, was the singular rule governing its chief priest, the Rex Nemorensis, "King of the Wood." Ancient sources, principally later Roman authors reporting an already old custom, describe the priesthood as held by a runaway slave who first proved his eligibility by breaking off a branch from a particular sacred tree in Diana's grove, and who then held office only for as long as he could defend it, since any successful challenger who killed him in single combat became the new Rex Nemorensis in his place. The office thus combined sanctuary, since escaped slaves were otherwise liable to recapture, with constant lethal risk, an arrangement ancient writers themselves treated as strange enough to comment on directly.
James Frazer opened his enormously influential comparative study of magic and religion, The Golden Bough, with exactly this custom, using the riddle of why a priesthood should be won and held by murder as the starting question for a wide ranging, if now largely superseded, theory linking sacred kingship, the death and renewal of a divine or semi-divine figure, and the fertility of the land across many cultures. Whatever the custom's precise origin, which remains debated among modern scholars of Roman religion, Nemi's Rex Nemorensis is one of the most distinctive institutions of religious violence documented anywhere in the classical Roman religious record, and it made an otherwise modest local sanctuary into one of the most discussed sites in the entire history of the comparative study of religion.