My theory is in order to save everyone from Arthur and Zelena in Camelot she had to battle Merlin but pretended that she destroy him and in order to cast the curse she tether part of Hook heart and his soul to the dagger to bring everyone back to Storybrook and she needed Rumple to pull Excalibur because he is a blank slate but also he knew Hook the longest and she doing all this to save Hook and destroy the darkness
After seeing the second sneak peak, I also wondered a similar thing.
I wonder where Lancelot is, though.
And it may not be the dark curse, just a curse built to look like one (hence the memory removal?). But that would beg the question as to why she would want to fake a dark curse, I must admit….
Keeper of Regina's bravery