His original name was Ciryaher. He was the fourth and last Ship-king.
Ciryaher began his reign reorganizing Gondor's armies and in 1050 T.A. he openly declared war on the Haradrim and defeated them. The Haradrim were forced to recognize him as their lord and king. After this victory he called himself Hyarmendacil, meaning South-victor.
Under his reign Gondor reached its greatest extent and power. The passes of Mordor were well guarded, so evil found no chance in Hyarmendacil's reign. No one dared to contest his power and Gondor was at peace for the rest of his long reign of 134 years.
He was succeeded by his lazy son Atanatar II.