Simon of Trent, a little boy who was made a saint because of his “Christian martyrdom” in an imagined act of barbarity supposedly committed by Jews.
The blood libel is the idea that Jews sacrificed children (especially Christian children) as part of their rituals. The blood libel was behind many waves of antisemitism and pogroms. It is absolutely false although to many who knew little or nothing about Jews it was both credible and incendiary.
The abhorrence of human sacrifice is one of the defining characteristics of Judaism. The Israelites were in all probability an offshoot of a Canaanite tribe which did indulge in ritual sacrifice of children but Judaism is built on a thorough rejection of such practices. The people of Europe were indulging in human sacrifice for a good thirteen or more centuries after the people of Israel had abandoned it, and of course they were symbolically eating Jewish flesh and drinking Jewish blood every Sunday.
Part of the reason for the blood libel persisting comes from Jewish behaviour in the time of the Crusades, several Jewish families followed the example of the people at Masada and killed their own children and themselves rather than be made to renounce their own primitive tribal superstitions at the point of a sword. I pity Christians reading this trying to work out whether those murderous Jews were heroes, martyrs, victims or villains. Just come across to the light, reject all superstition, you'll feel so much cleaner.