Arnau Mir de Tost

Arnau Mir de Tost (c. 1000 – after 1072) was a Catalan nobleman of Urgell, the lord of Llordà and viscount of Àger, and a major figure of the 11th-century Reconquista in Catalonia. Arnau is sometimes praised as the "El Cid of the Lands of Lleida" for his foundational role in reestablishing Christian rule in the region to the south of Urgell.

Arnau was born at Tost shortly after the year 1000. His father died when he was still young and he became attached to the court of Count Ermengol II. In 1031 he married Arsenda (or Ersenda) and purchased from the count the rights to the castle of Llordà, on the frontier with the Caliphate of Córdoba. To this end Arnau issued charters specifying the terms of colonisation; Montsec became the centre of a new Christian principality in the south of Urgell. For his primitive chancery Arnau employed an efficient scribe named Vidal, who brought the convenientia (convention) into common practice in western Catalonia, especially through the work he did drawing up conventions on behalf of Raymond IV of Pallars Jussà, the husband of Arnau's daughter Valença. Arnau had extended his influence into Pallars Jussà through this matrimonial alliance (1055), at which time Raymond had pledged him four castles as insurance against his breaking his marriage vows.

the mid-11th century by Arnau near Àger. Arnau controlled over thirty castles and numerous other agricultural settlements at the height of his power, justifying his assumption of the titles of viscount He built a fortified monastery, Sant Pere d'Àger, which still stands on a hill overlooking Àger.

In 1043–1044, Arnau became a hostage on behalf of his suzerain Ermengol III for 4,000 solidi to Raymond Berengar I of Barcelona, as insurance against Ermengol and Raymond Berengar's alliance against Raymond of Cerdanya. Arnau also fought for Raymond Berengar, conquering Camarasa and Cubells in his name. As early as 1050 he had extracted from Raymond Berengar a promise not to demand statica (right of lodging) from him. At the same time he was extracting other concessions out of Ermengol III, diminishing the comital rights (potestas, "power") over his castles.