When Merlin fell in love with Nimue, he used the Flame of Prometheus to reforge the Holy Grail into the sword Excalibur so he could use it to cute himself away from his magic and become mortal. Nimue gave in to Darkness and murdered Vortigan, becoming the first Dark One. Fearing Merlin would try to take away her powers, Nimue smashed Excalibur, severing the tip from the rest of the blade.

In order to keep Nimue's darkness from threatening the realms, Merlin fashioned the broken tip of Excalibur into a dagger, binding the Dark One to its control. This blade became known as the Dark One's Dagger. If a person killed the Dark One with the dagger, they would gain all of the power and knowledge, becoming the new Dark One. Whoever possessed the dagger could control the Dark One.

The name of the current Dark One is engraved on the blade. When the power switches to a new person, the old name vanishes and the new one takes its place.