One of the more unknown priestly colleges were the Arval Brethren, a college of twelve who's duty it was to perform the sacred rites required to ensure bountiful fertility of the fields. This priesthood is as old as Rome, yet an obscure one because it faded from use not long into the foundation of the Republic, but was later revitalized by the exacting Agustus who let no aspect of Rome's religion falter. It may have in fact lasted until the end of the Pagan religions in Rome at the end of the empire, and the Emperor himself was always counted as a member of the college.
What is known of their duties generally lay with the celebration of festivals to gods responsible for agricultural bounty. One example includes the significant three day festival to Dea Dia (Ceres) held in May which was announced from the Temple of Concord. Also later in the empire sacrificial rites called the Ambarvalia were performed on the traditional borders of Roman territory, a few miles out of the city walls, and also lustratio rites carried out for individuals and their appeals to the gods for bounty.
The Arval brothers kept their position in the priestly college for life and wore a corn or grain shaped garland fastened with a white ribbon around their heads to signify their important roles. There remains with us today one of their original chants to call upon the favor of Mars (in old Roman times also associated with agriculture), and is the oldest record of a Roman incantation:
Help us Lares! Help us Lares! Help us Lares!
Marmar, let not plague or ruin assail more folk. Marmar, let not plague or ruin assail more folk. Marmar, let not plague or ruin assail more folk.
Be full satisfied, fierce Mars. Leap the threshold. Halt. Beat the ground. Be full satisfied, fierce Mars. Leap the threshold. Halt. Beat the ground. Be full satisfied, fierce Mars. Leap the threshold. Halt. Beat the ground.
By turns call on all the gods of Sowing. By turns call on all the gods of Sowing. By turns call on all the gods of Sowing.
Help us, Marmor! Help us, Marmor! Help us, Marmor!
Bound, bound and bound again, bound and bound again!
Sources: LacusCurtius, and 'Roman Religion' by Valerie M. Warrior