Nemi is a small crater lake in the Alban Hills near Rome, famous because of the Arician grove nearby, which in ancient times held a priest-king, who was killed by his successor. The origin and meaning of this custom was the spur to Sir James Frazer for his monumental study of mythology, The Golden Bough.
On the shores of Lake Nemi, Latin name Lacus Nemorensis, was a temple of Diana. For that reason it was also called Speculum Dianae, Diana's mirror. In this grove was the golden bough, immortalized in Turner's painting. It was a haven for runaway slaves. One such was presented with a challenge by Diana, to kill a champion she had put in the grove. He pulled away a huge branch from an oak, and defeated the champion, for which Diana rewarded him with the title Rex Nemorensis. The name is related to the Greek nemos 'grove'.
In the sacred grove there grew a certain tree round which at any time of the day, and probably far into the night, a grim figure might be seen to prowl. In his hand he carried a drawn sword, and he kept peering warily about him as if at every instant he expected to be set upon by an enemy. He was a priest and a murderer; and the man for whom he looked was sooner or later to murder him and hold the priesthood in his stead. Such was the rule of the sanctuary.
A candidate for the priesthood could only succeed to office by slaying the priest, and having slain him, he retained office till he was himself slain by a stronger or a craftier. The post which he held by this precarious tenure carried with it the title of king ; but surely no crowned head ever lay uneasier, or was visited by more evil dreams, than his. For year in year out, in summer and winter, ill fair weather and in foul, he had to keep his lonely watch, and whenever he snatched a troubled slumber it was at the peril of his life. The least relaxation of his vigilance, the smallest abatement of his strength of limb or skill of fence, put him in jeopardy grey hairs might seal his death-warrant.
-- Frazer
During drainage of Nemi in 1930, two Roman ships were discovered, probably pleasure barges of the emperor Caligula. In 1944 these were burned by the retreating German army.
The King of the Wood
The still glassy lake that sleeps
Beneath Arica's trees -
Those trees in whose dim shadow
The ghastly priest doth reign
The priest who slew the slayer
And shall himself be slain
-- Macaulay