This work highlights and describes the specific way in which fictional discourse in Spanish gradually took shape and developed in the rewriting of history in the first half of the 13th century. The battle of Atapuerca (1054) is a milestone in the history of the Kingdom of Navarre. Since its first historiographical version preserved in the Historia silense (c. 1118-1126), the account of the battle of Atapuerca underwent modifications, amplifications and several recasts in its extensive dispersion in time and space. In the first place, the work presents a comparison between different Latin versions (mainly the Silense, the Chronicon mundi and De rebus Hispaniae) and several Castilian versions (the Versión crítica de la Estoria de España, the Crónica sanchina, the Crónica de Castilla and the Crónica general de 1344) of the battle of Atapuerca. Secondly, a detailed analysis of a late version of the battle is carried out taking into account the evolution of narrative prose in medieval Spanish. This version adopts the typical narrative forms of the fazaña, a minor Castilian legal genre. The work focuses at this point on the peculiar intersection between the legal mould provided by this short narrative form and the traditional historiographic matter. In the treatment of historical matter and in the peculiar presentation of motifs and characters in this episode, the fazaña shows striking features that many critics have identified as fictional or even novelesque.