Dark One Dagger

History
The Dark One's Dagger has a curse that grants incredible magical powers to whoever's name is inscribed on its blade; this person is known as the "Dark One." Unfortunately, a person who possesses the dagger can control the Dark One and his powers. At some point, the Duke gained possession of the dagger when it had Zoso's name inscribed on it, and he was able to control the Dark One for his bidding. He kept the dagger hidden in his home.

The curse passes from one Dark One to the next when someone uses the dagger to kill the person whose name is on the blade. Zoso, not wanting to be the Dark One anymore, tricks Rumplestiltskin into stealing the dagger from the Duke's home and using it to kill Zoso. When Zoso dies and Rumplestiltskin gains the powers of the Dark One, his name replaces Zoso's on the blade.

When Baelfire expresses concern for the way that the curse has changed his father, Rumplestiltskin pulls out the dagger, now bearing his name, and explains to his son that the only way to get rid of the curse is for someone to kill him with the dagger. Rumplestiltskin uses extreme caution in protecting the dagger lest he become enslaved to a master the way Zoso had or lest someone steal it and kill him so they can take his powers.

When the Dark Curse hits Fairytale Land, Rumplestiltskin brings the dagger with him into Storybrooke. Although the dagger loses its magic, it still has his name inscribed on it. As Mr. Gold, he keeps it in his possession in his store until Emma comes to town. Then, he takes it out into his land in the forest and hides it in the ground for safekeeping.

August tricks Mr. Gold into revealing the location of the dagger and digging it up. Believing August is Bae, Gold hands him the dagger. Once he's in possession of the dagger, August tries to use it to control Mr. Gold, to make him use his magic to save him. Mr. Gold realizes August isn't Baelfire, because his son would never use the dagger against him, and because his son knew that the dagger has no effect in a land without magic. Mr. Gold holds the dagger to August's throat, but decides not to kill him.

Trivia

 * The importance of the knowing name on the dagger is a play on the original Rumpelstiltskin fairytale, when he made a deal with the miller's daughter to discover his name.