Cantiga 108 of the Cantigas de Santa Maria of Alfonso X

Merlin and the Jew
Merlin and a Jewish sage were engaged in a debate.
The Jew refuted the Incarnation and Merlin defended the doctrine.
Merlin grew angry and prayed to the Virgin to reveal the truth. He asked her to cause the son of the Jew to be born with his head on backwards.
When the child was born he was disfigured as Merlin had requested.
The father tried to kill the baby.
Merlin saved the child and used him to convert the Jews.
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List of Cantigas de Santa Maria