This is a phrase or name denoting the main character in the Mystery Tradition at Lake Nemi in Italy. Otherwise known as Rex Nemorensis who is the guardian of the Sacred Grove of goddess Diana. Traditionally he was challenged through ritual combat every year to fight for the right to rule. This demanded that his opponent break a branch from the sacred tree in the grove. According to legend, only a man possessing great inner and outer strength would be able to do this.
This resembles a rite of initiation. The legend of Rex Nemorensis is similar to Aeneas who had to break the golden bough from the tree of the Underworld in order to complete his sacred quest. Aeneas encounters Charon the guardian who refuses to let him cross the Underworld lake. Charon and the King of the Woods are parallel figures as the latter is guardian of Lake Nemi. The King of the Woods also is referred to by other names such as a type of Green Man figure, in Italian Witchcraft as the Hooded One since he is covered with greenery of Nature, and in Diana’s sacred grove at Nemi he is called Virbius. A.G.H.
Source: 78, 206-207.