Supposedly, lepers acting on the orders of Jews poisoned the waters, with the purpose of poisoning the Christian population of Europe. As a result, the king ordered the Jews arrested.
Eventually, the truth came out, and the king ashamed of himself for arresting innocent people changed the accusation to be, that they were arrested in order to convert to Christianity. The Jews refused, and they were therefore tortured. 15,000 holy souls were then burned at the stake!
Source: Shevet Yehuda §43
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