It's the origin story of Van Helsing's little antique thing (I still don't know what it was called) that he uses to mesmerise Jonathan. Van Helsing picks out the most unique item that will both fascinate and confuse Seward, so that he can test out his newfound skills on him first. It was also a haunted antique, and his research into how to cleanse it of that little issue led to his interest in more matters than just mesmerism.