Board Thread:Spoilers!/@comment-1916997-20151017160320/@comment-24674051-20151029042825

What if they somehow "Divide" the darkness. Swan has some, Nimue has some, maybe some goes back to Rumple? Maybe the problem is not that there is darkness in the realms, but that Merlin (and possibly Nimue?) tried to gather it all together in one spot to destroy it. So the darkness can not be destroyed, but it can be spread back out, and the larger pools will still need to be somewhat controlled, but will be controllable in smaller quantities.

But it takes a fully reforged Excalibur to split the darkness, and with the darkness split, maybe excalibur is destroyed. (just another wild theory)