Board Thread:Show-Related Questions and Answers/@comment-5279400-20171227043948/@comment-25375217-20171227045001

The writers were asked that before Season 5 ended and they said we would find out in Last Rites. Considering we saw the Souls attacking Arthur and Hook in that episode, presumably that's what they're referring to.

But all that tells us is that the souls can escape... to drag other souls in to be tortured with them. (Misery loves company, I guess.) The real question is, will they ever regain sentience and escape permanently?