The Oath of Eorl was the promise of unending alliance between his people and those of Gondor. It was spoken by Eorl the Young atop Amon Anwar after he and his people were given the lands of Calenardhon by Cirion the twelfth of the Stewards of Gondor.

The Oath of Eorl was spoken in the tongue of the Éothéod and this is how it was translated:

“Hear now all peoples who bow not to the Shadow in the East, by the gift of the Lord of the Mundburg we will come to dwell in the land that he names Calenardhon, and therefore I vow in my own name and on behalf of the Éothéod of the North that between us and the Great People of the West there shall be friendship for ever: their enemies shall be our enemies, their need shall be our need, and whatsoever evil, or threat, or assault may come upon them we will aid them to the utmost end of our strength. This vow shall descend to my heirs, all such as may come after me in our new land, and let them keep it in faith unbroken, lest the Shadow fall upon them and they become accursed.”

After the War of the Ring, the king of the third line of the Rohirrim, Éomer, took again the Oath of Eorl and promised his people's service to the new King of the Reunited Kingdoms, Elessar.
Encyclopedia entry originally written by Nienna-of-the-Valar