Board Thread:Spoilers!/@comment-27060727-20170106065555/@comment-27060727-20170204232736

According to Kat, Isaac is returning in 6x16. http://katmtan.tumblr.com/post/156818206997/is-isaac-in-6x17

This is when they did a lot of filming at that mansion, where all of those storybooks were initially found in S4, so there's a possible connection.