There's something odd about Shiren. In chapter 5, she pretended not to know what the cloud of darkness was, even though it was supposedly strong enough to be visible by that point. Their relationship seemed to be too saccharine for her to hide the curse and then coldly demand him to follow her to Inbe, and ultimately kill her all of a sudden, and on the nick of time. It might have taken more than just visible form, now that all of its pieces are gathered within her.
Additionally, they were being followed on their way to the shrine, but by whom remains to be seen.