Louis was feeling adventurous. He wanted to share his energy with someone, and when he realized that the moon was full tonight, he knew what he could do. Remus had told him he was taking Wolfsbane and would be in the forest. That meant he had the opportunity to see a real werewolf without any real danger, which definitely counted as an adventure. He'd have to be careful that Remus didn't see him though. He probably wouldn't be too pleased.
So that was how he found himself outside Bellatrix's door. Who better than to share this sort of undertaking with? He thought he could maybe surprise her. Or impress her. Whichever one took. It took him a while before he recognized the bench in front of the right house. He had thrown on black trousers and a black shirt, thinking it might be easier to sneak about in dark colors. And it was slightly more fun.
He reached out and knocked several times on the door, hoping she was in.