As late as the 1926, and perhaps even up to the eve of World War II, on Fridays, in a small church on the Jewish street, a few meters off the main market square in the city of Poznań, the traditional song Kyrie Eleison, God Have Mercy, Christ Have Mercy, was replaced by a song on Jewish desecration of the host in Poznań in 1399. It was a legend, for there is no record of the case; no court room drama, for the trial never took place. The drama was elsewhere: the set was the city, its geography, and its religious composition. The protagonists were Carmelite friars who were trying to build in their church of Corpus Christi, and then revive, a eucharistic cult around a legend of host desecration by the Jews. Poznań’s host desecration legend did not just propagate a local eucharistic cult in the post-Reformation era, it also promoted contested tenets of the Catholicism and served as polemics against the Protestants. More practically, the legend also provided the Carmelites of Poznań whose church of Corpus Christi was located in a remote swampy area outside the city walls with the opportunity to obtain a property they desired within the city—the very site that by the eighteenth century would become the church of the Most Holy Blood of Christ. All this took place in a very specific time and place, as dogmas were contested, jurisdiction challenged, and competition between Jews and Christians increased. And the legend that the miraculous hosts had been desecrated by Jews served to affirm the sacredness not only of the Eucharist but also of the Catholic space.