After blackmailing Lauderback successfully, Carver headed over to Lydia's house to tell her the good news.
She then informed Carver that, in a stroke of "luck," Crosbie had taken up with Anna. This meant she could get them easily out of the way that evening, as she had sent up a decanter of liquor to them that was laced (ostensibly with laudanum).
Francis said he was going to go up. It seems he planned to spirit the presumably drugged out Crosbie away from there and "tap him on the head."
He snuck upstairs and tried to do just that, but when he entered, Crosbie (who wasn't drugged) bashed him over the head.
He asked Carver who he was, and Carver said he was Crosbie Wells. When Crosbie asked where his gold was, Carver explained that "Crosbie" himself had shipped it offshore.
Crosbie then knocked him out cold and used a knife to carve a long C-shaped wound into his face.
He then told Anna to drink up from the decanter so she could claim she had slept through the whole thing. She went and got a gold nugget he had given her when they first met—to help her get away from Lydia—and gave it to him to help him get away.