The rex sacrorum was a feature of Italic religion and possibly also Etruscan. The title is found in Latin cities such as Lanuvium, Tusculum, and Velitrae. At Rome the priesthood was deliberately depoliticized; the rex sacrorum was not elected, and his inauguration was merely witnessed by a comitia calata, an assembly called for the purpose. Like the flamen Dialis but in contrast to the pontiffs and augurs, the rex was barred from a political and military career. He was thus not a “decayed king”; rather, after the overthrow of the kings of Rome, the office of rex sacrorum fulfilled at least some of the sacral duties of kingship, with the consuls assuming political power and military command, as well as some sacral functions.
Source: Rex Sacrorum – Wikipedia