She found her way into a small enclosure where the smaller creatures were kept. She found a terrarium with a number of bowtruckles relaxing inside it. She only wanted a little practice. No harm, really. Just to see how they'd react. She snatched one out carefully and put it on the floor quickly, avoiding its bite by surprising it. She drew her wand, looking at the stick-like thing dispassionately, and spoke in a bare whisper, "Crucio."
The bowtruckle shrieked and rolled on the rough plank floor, small talons grasping for purchase. Bellatrix' stared at it, then levelled the curse upon it again. And again. And again. Until it ceased to move. She squatted down and poked at it with her wand. "Fascinating," she said aloud. It was still alive, but limp and listless. She picked it up with two fingers, holding it away from herself. "Lost your wits, little twig?" she asked it. "What a terrible pity. But it I assure you it's for the greater good."
She tossed it back in the terrarium, content with her experiment. She would have to try something bigger next time. She strolled out of the enclosure, and headed back towards the castle, a slight spring in her step.