by Meia Geddes

fetial \FEE-shuhl\, adjective:

Concerned with declarations of war and treaties of peace.

When a just and rightful war was declared upon a foreign enemy—and were there any other kinds of wars?—a special fetial priest was called upon to hurl a spear from the steps of the temple over the exact top of the ancient stone pillar into the earth of Enemy Territory.
— Colleen McCullough, The First Man in Rome

He struck his treaties with foreign princes in the Forum, sacrificing a pig and reciting the ancient formula of the fetial priests.
— Edited by John Carew Rolfe, Suetonius

Fetial comes directly from the Latin word fētiālis, which referred to a member of the Roman college of priests who were representatives in disputes with foreign nations.