In the days of Maimonides — Rabbi Moshe ben Maimon, 1138-1204 CE — evil decrees were issued against the Jews of his city. The laws were designed to humiliate.
If a gentile were so much as touched by a Jew, the gentile's garment was to be burned and the Jew was to pay heavy damages. The gentile also had to bathe seven times in ritual purification from the "defilement" of Jewish contact. If a gentile beat a Jew, the Jew was required to compensate the attacker for the effort of beating him.
A case came before Maimonides in which a Jew had been beaten by a gentile, and the court demanded the Jew pay for the beating. Maimonides, serving as judge and as counselor to the king, paid the sum himself: twenty gold pieces for the gentile's burned clothes, seven more for the inconvenience of bathing in winter.
Then Maimonides began a small counter-performance. He arranged for two Jews to quarrel at the city gates and to bring their dispute to him for judgment. The question they posed was this: a mouse had fallen into a cask of oil belonging to one of them, and a gentile had briefly touched a cask of wine belonging to the other. What was to be done?
Maimonides issued his ruling in a booming voice, where the gentile crowd could hear: the mouse-tainted oil could still be used for lamps or soap, for a mouse does not defile beyond repair, but the wine touched by a gentile must be poured down the drain, destroyed completely, because contact with a gentile had made it irrecoverable.
The gentiles understood what the ruling implied about them. They exploded in fury. The king ordered Maimonides brought to the city square and burned.
When the guards came for him, Maimonides pronounced the Shem HaMeforash, the Ineffable Name. His form changed. He became a lion, enormous and terrible, and in the confusion he slew seventy thousand of the soldiers arrayed against him, sparing only the king himself — who had begged his life. The king, humbled, revoked every one of the evil decrees. Israel rejoiced (Gaster, Exempla No. 344).
The story is a legend, not a historical record. Maimonides never turned into an animal. But the Jewish people, telling this story under the weight of medieval persecution, needed to imagine a sage who could answer cruelty with a strength that overflowed human form. The moral is not that the righteous become lions. The moral is that the righteous become more than themselves when the community is truly in danger.