This is, in a nutshell, the surviving legend of the rex Nemorensis: the priesthood of Diana at Nemi was held by a person who obtained that honour by slaying the prior incumbent in a trial by combat, and who could remain at the post only so long as he successfully defended his position against all challengers. However, a successful candidate had first to test his mettle by plucking a golden bough from one of the trees in the sacred grove.

Source: Rex Nemorensis – Wikipedia