Bellatrix hadn't really been up to anything interesting since her little conversation with Fleur Weasley, the pregnant woman. Sure, she had interrupted students and professors alike, thrown insults about (as had they), but nothing really stood out in her mind. She was bored. She had explored the entire castle, if such a thing were possible, and she was running out of people to render miserable. The youngest Weasley boy looked like a potential target, the way he was moping about, as was Granger, but she really couldn't have cared less. She didn't want them getting together on account of her pointing out they hated each other. Sometimes, it was best to stay out of the way and let the horrors occur all by themselves.
It had never occurred to her to leave the castle until now. Something wanted... the kill. Something was hungry for blood, and Bellatrix couldn't help but be attracted by the feeling.
She heard the growl, and knew she was following a good trail. "Fenrir Greyback," she said, trying to hide her surprise as she came up behind him. She didn't suppose people sneaked up on the... thing often, but without a body to hear or scent, she supposed she might have this time.