<p>This study presents a new reading of the <i>Perlesvaus</i>, an anonymous thirteenth-century Old French Grail romance bizarrely structured around an Arthurian restaging of the battle between the Old and the New Law. I construe this hyper-violent, phantasmagorical text as a profoundly significant work of “trauma fiction” encoding a hitherto-unrecognized crisis of religious ethics and identity in Western Europe in the first half of the thirteenth century. Combining literary and historical analysis and drawing on current trends in trauma studies, I tie what I term the “deranged discourse” of the <i>Perlesvaus</i> to the brutal onset of internal crusading in southern France (the papal-sponsored Albigensian Crusade, 1209-29), making the case that the collective trauma staged in its narrative perturbations was a contributing factor in the well-documented worsening of Western European antisemitism during this period. One key analytical construct I develop is the “doppelganger Jew”—personified in the <i>Perlesvaus</i> by its schizoid authority figure, Josephus, a conflation of first Christian priest and first-century Romano-Jewish historian—who functions as an uncanny embodiment of powerful, unacknowledged fears that Christians were losing their spiritual moorings and reverting into reviled, scapegoated Jews. Traces of this collective trauma are explored in other contemporary texts, and one chapter examines how the fourteenth-century <i>Book of John Mandeville</i> revives similar fears of collapsing Judeo-Christian identity and unfolds under the sign of the doppelganger Jew.</p>