Rex Sacrorum, the king of sacrifices.
The Rex Sacrorum (Latin “king for the sacred”), also known as Rex Sacrificulus was one of the highest Roman priests.
In February, the rex sacrorum and the Flamen Dialis jointly distributed the Februa, a cultic brushwood broom with which fields, meadows and houses were ritually cleaned at the beginning of field cultivation.
In the time of the Roman monarchy, it was the king who made the “great sacrifices”, his person being reputed to be the purest in the whole empire and the most pleasing to the gods.
The Romans constituted as a republic did not become less superstitious, because they attributed their prosperity to the care they took in their worship, they were afraid of displeasing heaven by no longer using the hand of a king for holy things: this is why they wanted to keep at least the word or the title by instituting a king of sacrifices; Rex Sacrorum.
The Rex Sacrorum belonged to the College of Pontifices and was directly subordinate to the Pontifex Maximus, although in the traditional cultic hierarchy it stood above the flamines maiores and the pontifex maximus.
Designated by the pontiffs and augurs, he was created by the people assembled in centuries, and always drawn from the order of the patricians, and from the names of the eldest; he was charged with doing exactly what the King once did in religion; he had no other office in the Republic and was free from all civil and military obligations. He had more or less the same privileges as the pontifices, but in contrast to these he was subject to several serious official restrictions. The most important was that he was not allowed to hold political office, which made the office not very desirable in the late Republic.
Source: L’antique Rome, ou, Description historique et pittoresque de tout ce qui concerne le peuple romain, dans ses costumes civiles, militaires et religieux, dans ses moeurs publiques et privées depuis Romulus jusqu’à Augustule: ouvrage orné de cinquante tableaux par Jacques Grasset de Saint-Sauveur. Paris: Chez Deroy, libraire, rue du Cimetière-André, no. 15, près la rue Haute-feuille, et chez les principaux libraires de la République 1796.