This work highlights and describes the specific way in which fictional discourse in Spanish gradually took shape anddeveloped in the rewriting of history in the first half of the 13th century. The battle of Atapuerca (1054) is a milestone inthe 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 extensivedispersion in time and space. In the first place, the work presents a comparison between different Latin versions (mainlythe Silense, the Chronicon mundi and De rebus Hispaniae) and several Castilian versions (the Versión crítica de la Estoriade 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 narrativeprose 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 formand the traditional historiographic matter. In the treatment of historical matter and in the peculiar presentation of motifsand characters in this episode, the fazaña shows striking features that many critics have identified as fictional or evennovelesque.