This thesis provides evidence that a particular "topos" from the high Baroque's exalted style, the so-called chaconne bass, made a profound impact on a considerable body of Mozart's compositions from the last ten years of his life in Vienna. After identifying the topos in the first chapter, a detailed study in chapter two shows how Mozart's faith in the extraordinary emotional power carried by this topos was enough for him to work it into all of the completed keyboard fantasias. Chapter three illustrates that an understanding of the chaconne bass and its unmistakable association with the fantasia style can shed new light on three of Mozart's most enigmatic compositions from his final period, K. 465, K. 491, and K. 527.