Explanation:
Sirius Black escaped from Azkaban by using his Animagus form, a large dog.
He’d been imprisoned in Azkaban for murdering thirteen people with a single curse. The prison guards were very thorough in making sure that no one could escape—but they didn’t realize that Sirius had been living as an Animagus for years before his arrest. He simply transformed into his dog form and ran away through the bars of his cell, which were too small to fit him as a man.