Douglas Harper's Etymology Dictionary
1530s, "pertaining to the Fetiales," the Roman diplomatic corps, a college of 20 priests whose duty was to act as heralds and maintain the laws of war, from Latin fetiales "speaking, negotiating, diplomatic," which is of unknown origin.
Wikipedia
A fetial (Latin plural fetiales) was a type of priest in Ancient Rome. They formed a collegium devoted to Jupiter as the patron of good faith.
The duties of the fetials included advising the senate on foreign affairs and international treaties, making formal proclamations of peace and of war, and confirming treaties. They also carried out the functions of traveling heralds or ambassadors. One of the fetiales, named pater patratus, was appointed as the fetials' spokesman.
The first mention of the fetials by Livy occurs in the context of the war between Alba Longa and Rome, during which the Roman king Tullus Hostilius appointed M. Valerius as a fetial and Sp. Fusius as pater patratus, for the purpose of binding Rome and Alba Longa by a treaty.
According to Livy, the ritual by which the fetials were to declare war, the ritual of rerum repetitio, was introduced to Rome by Ancus Marcius, borrowing on the traditions of the Aequicolae. However he had already described the ritual actions of the fetials when recording the wars of Tullus Hostilius. Thus some scholars think the mentions of the Aequi may be a misinterpretation due to a folk etymology connecting Aequi to aequus, the Latin adjective for fair. On the other hand ancient sources support the tradition that the priesthood was created under the influence of Aequian king Ferter Resus.
Usage examples of "fetial".
The Fetial put the formal question to Tullus: "Do you, King, order me to make a treaty with the Pater Patratus of the Alban nation?
On the king replying in the affirmative, the Fetial said: "I demand of thee, King, some tufts of grass.
It was customary for the Fetial to carry to the enemies' frontiers a blood-smeared spear tipped with iron or burnt at the end, and, in the presence of at least three adults, to say, "Inasmuch as the peoples of the Prisci Latini have been guilty of wrong against the People of Rome and the Quirites, and inasmuch as the People of Rome and the Quirites have ordered that there be war with the Prisci Latini, and the Senate of the People of Rome and the Quirites have determined and decreed that there shall be war with the Prisci Latini, therefore I and the People of Rome, declare and make war upon the peoples of the Prisci Latini.