"Firenze!" Bane thundered. "What are you doing? You have a human on your back! Have you no shame? Are you a common mule?"
"Do you realize who this is?" said Firenze. "This is the Potter boy. The quicker he leaves this forest, the better."
"What have you been telling him?" growled Bane. "Remember, Firenze, we are sworn not to set ourselves against the heavens. Have we not read what is to come in the movements of the planets?"
Ronan pawed the ground nervously. "I'm sure Firenze thought he was acting for the best," he said in his gloomy voice.
Bane kicked his back legs in anger.
"For the best! What is that to do with us? Centaurs are concerned with what has been foretold! It is not our business to run around like donkeys after stray humans in our forest!"
Firenze suddenly reared on to his hind legs in anger, so that Harry had to grab his shoulders to stay on.
"Do you not see that unicorn?" Firenze bellowed at Bane. "Do you not understand why it was killed? Or have the planets not let you in on that secret? I set myself against what is lurking in this forest, Bane, yes, with humans alongside me if I must." (Harry Potter and the Sorcerer's Stone)
How can I figure out what that refers to in this sentence?