Sancho Alfónsez (or Adefónsez) (c. 1093 – 29 May 1108) was the only son of King Alfonso VI of Castile and León; his mother was the Moorish princess Zaida.
His death, on his first recorded military expedition, precipitated a succession crisis that ended with the accession of his elder half-sister Urraca and her husband, Alfonso the Battler, already King of Navarre and Aragon, to the throne of Kingdom of Castile-León.
[4] Though illegitimate, his birth must have dashed the hopes of Raymond, the Count of Galicia and son-in-law of the king, who, according to the Chronicon Compostellanum, had been promised the kingdom.
[6] Another unreliable charter, this one dated to 12 January 1102 (though it says 1110), names Sancius filius Imperator ("Sancho, son of the emperor") among its witnesses, but it contains interpolations.
Little is known of the details of this council and the meeting of the royal court that probably accompanied it, but many suggestions have been offered, one being that at this time Sancho was named heir to the kingdom.
[14] On 16 March 1104 he confirmed a grant to the bishop of Oviedo that is the first known appearance of his half-sisters Sancha and Elvira, the daughters of Alfonso's new queen, a Frenchwoman named Isabel.
[17] Sancho does not reappear until 19 March 1106, when he confirmed his father's grant to the church of Oviedo, made at Sahagún, the court's favourite resting place.
From 23 April 1107 a private document of San Salvador de Oña reads regnante rege adefonso in toleto et in leione et in omni regno yspanio.
[22] Claudio Sánchez-Albornoz, followed by Bishko, redated the charter to 1105 on the basis of the Historia Compostelana, a date which would lend support to the theory of a pacto sucessório (pact of succession) between Henry and Raymond in the spring of that year.
[23] The death of Constance, the birth of the illegitimate Sancho, and Alfonso's quick remarriage to an Italian named Bertha had altered the state of the succession in 1093.
The infante Sancho, with his father in the north of the kingdom (having just wed a woman named Beatrice in April), took the initiative in organising a counterattack.