Dumbledore hinted to Hermione Granger and Harry Potter to go back in time to save, both, Buckbeak the Hippogriff(or "hippogriph")and Sirius Black from death. To do that, Hermione used her "Time-Turner" necklace(which she used all term to attend her numerous amount classes), gave it three turns and back in time they went. Harry and Hermione saves them by untying Buckbeak before the executioner came and releasing Sirius Black from Pro. Flitwick's offise. To let Sirius leave the grounds, they had to give Buckbeak to Sirius. They both lived in that book.