User blog comment:HazelQuill7445/Awesome fan art/@comment-4010415-20170805071949/@comment-34633327-20170806073738

So, from what I can tell, it seems Weiss and Ruby were on a mission, the two got seperated, and Weiss was approached by "Angel". They did battle and the black markings that touched Weiss began affecting her, making her more psychotic and and amplifying her power. The markings simultaneously began to show Weiss some of Angel's memories, and soon Angel defeated her. She left Weiss with Ruby, who was also knocked out somehow, and the markings still affecting her caused Weiss to try and kill Ruby, only to be stopped by a mysterious newcomer. As for Angel, she seems to have been a rich girl who lived in a gilded cage much like Weiss did, the illusion of freedom and all that. Someone approached her and gave her those black markings, causing her to slaughter her family (There is a split seen of her covered in them smiling like a maniac) and break free of her cage. So I'm guessing that isn't actually her semblance but rather something controlling her, which can infect others that touch it, something Weiss kinda realised hence her dancing around the markings.