Mind control

Lucas had always looked down on Bella. So when she tried to touch him he did not try to protect himself. But then he could feel Bella crawling into his mind and trying to control him. He had never thought that demons would have the power to crawl inside his mind. 

He thought that it was because he was in the form of a dead being. So he took his human form but that was the biggest mistake he had made. Her control on his brain increased. As if he was growing weaker every second, she could look into his memories.

Even his darkest secrets were for her to see and if that was not enough, she forwarded her hand and Lucas himself had put his scythe on her hand. She had complete control over his brain and body which came as a shock for both Alex and Lucas who had never thought that Bella was that string.