Board Thread:Spoilers!/@comment-2153169-20151015134011/@comment-6175354-20151110042753

Eskaver wrote: Well, if he gains Merlin's powers and et cetera, I think I would have to stop being so positive about the show. What part of temporary, escapes your comprehension?

That picture implies, that Hook's soul was tethered to the sword, back in Camelot. The Holy Grail, a.k.a. Excalibur, was the source of Merlin's magic.