In Harry Potter's first year at Hogwarts, Draco Malfoy reported them for sneaking out at night, so they were all punished with a trip to the Forbidden Forest.
It was a rather strange punishment, since there were many ways to serve detention, especially when Snape and Filch never lacked work to assign.
There, they encountered a dead Unicorn and a "beast" crouched over it, drinking its blood. At first, Harry and Malfoy were both stunned. It was not until Fang began barking furiously at it that Malfoy let out a terrible scream and bolted. It was Fang and Malfoy's cries that drew Firenze over, and Firenze then saved Harry.
Even among Centaurs, drinking Unicorn blood could not be considered "pure." Firenze had sensed even then that something was about to happen.
Compared with the matter that concerned his kin—letting a human ride on his back—Firenze believed it was more important to get that "Potter family child" out of the Forbidden Forest.
To the Centaurs, the Forbidden Forest was "theirs." Any "thing" that trespassed without permission had to be driven out or resisted. In the past, the Centaurs' main precaution had been against Acromantulas, but now there was another such "thing." As long as he could protect the forest, Firenze did not mind allying himself with humans. He had sought out Pomona tonight to thank her for sending the interns to clear away the Acromantula webs, and the reason he had found Pomona by the Black Lake was also because of "the omens in the stars."
Fateful Stars were often misread. Even Centaurs were not immune to mistakes, but these astrologers had seen the signs revealed by the movements of the planets, so the chances of being right were greater. Even so, they had no intention of getting involved.
A compass