In 1944 Third Age the Wainriders invaded Gondor from the east while their allies, the Haradrim invaded Gondor from the south. Ondoher and his eldest son Artamir led the main army close to the Black Gate where they clashed with the Wainriders. The left wing of Ondoher's army was commanded by Adrahil, the Prince of Dol Amroth. The right wing was commanded by Minothar, the captain of Gondor's armies. The center was commanded by Ondoher himself.
In the ensuing battle Ondoher was slain together with his eldest son and heir Artamir, and Gondor defeated. The rearguard of the army defended Ithilien while Adrahil got help from General Eärnil who had defeated the Haradrim.
Although Eärnil finally defeated the Wainriders it was learned that Ondoher's second son Faramir was slain with his father while law required that at least one heir was left behind.
No-one with a valid claim to the throne could be found, and Arvedui of Arthedain claimed the throne. Although Arvedui had a strong claim by his way of his wife Fíriel, who by ancient Númenórean law now should have become Ruling Queen, the throne was granted by Steward Pelendur to the general Eärnil, who would reign as King Eärnil II.
Other Versions of the Legendarium
Ondoher was spelled Ondohir in the first edition of The Return of the King.