Board Thread:General Discussion/@comment-25926288-20151207015752/@comment-24674051-20151208152139

True, but...

Gold did something with Excalibur that allowed him to get the dagger back, which means that, at some point, Excalibur became unforged.

And Excalibur (at least the lower blade and hilt) was disintegrated when Emma plunged it into Killian, so (assuming that Emma has the Darkness originally tethered with Excalibur lower blade) Emma would no longer be tethered to anything. and that may be something that either Gold does not anticipate, or something that he DOES anticipate that concerns him greatly.

Granted, this is a great deal of speculation, and is probably totally off the mark.