Board Thread:Spoilers!/@comment-24304025-20150202171011/@comment-24674051-20150206144042

How people know of his weakness. While it's never been said, it's not that hard to figure out. Other DO's have been controlled in the past, so it would stand to reason that there is some legend in the EF that states that in order to control the Dark One, you must control the mystical dagger (or something to that effect).

And it would not surprise me if some scholarly person (**Cough*Sorcerer*Cough**) documented some of the other aspects of the dagger and the dark one, including the vault, and how the power passes to a new dark one when you kill them with the dagger. Obviously, Belle was able to do some research on the vault, even in Storybrooke (She was able to identify the mark on Neal's hand right before he died). Definitely not common knowledge, but there for the right person to find. Who knows, maybe somewhere is written how to break the curse of the DO?

And of course, once someone finds the information, it probably won't take too long for others to find it. Killian got the information from Bae, Cora from Killian, Regina from Cora, Emma from Gold (in NYC), Charming and Snow from Emma. (Shall I continue).