Oath of Eorl
The Oath of Eorl is the alliance sworn between the Middle-earth nations of Rohan and Gondor. The Oath also stated that the province of Calenardhon will be forever of Rohan, and the borders of that kingdom were established.
Also agreed was whenever help was needed, the two kingdoms would aid each other. The signal that help was needed was the Red Arrow (which means danger). This arrow was brought to King Théoden of Rohan by a messenger of Gondor in The Return of the King while Minas Tirith was besieged by Sauron. For faster communication between the two allies, the warning beacons of Gondor were used. These beacons were placed on seven hills along the northern ridge of the White Mountains.
History of the Oath
The Oath was first sworn in T.A. 2510 by Cirion, the twelfth Ruling Steward of Gondor, and by Eorl, King of the Eotheod (later first King of Rohan) following the Battle of the Field of Celebrant. They swore their oaths on the Halifirien hill at the site of the tomb of Elendil.
Eorl spoke his oath in his own language (Rohirric). In addition to his oath, Cirion spoke an invocation in Quenya and the Common Speech, calling the Valar and Eru to witness the oaths that were taken. Eru's name was very rarely invoked in oaths, and indeed had not been since the time of the Last Alliance. Among people of Numenorean descent, it was held that only the King could call upon Eru, but Cirion had that authority as Steward and regent for the King. (Unfinished Tales: Cirion and Eorl)