The Dark One's Dagger, also known as the "Dark One Dagger", is a magical but very dangerous dagger and the recurring object in the ABC television series Once Upon a Time. This arcane weapon carries the curse of the Darkness, and allows anyone who holds it to control the Dark One , making them virtually unstoppable. If a person holding the dagger uses it to kill the current Dark One, they themselves will become the Dark One and with the title as the "Dark One" comes an immense innate knowledge of all forms of magic, especially dark magic, as well as an virtually unlimited amount magical powers, making the new Dark One the most powerful sorcerer in all the realms (rivaled only by Merlin who has the same form of magic, but light magic instead of dark magic).
It is said that if someone gained hold of the dagger and owns it, he or she will control the Dark One.
As stated above, if someone kills the Dark One with this dagger, he or she will then become the Dark One upon being immediately possessed by the Darkness and takes their magic powers.
The dagger is actually the tip of the legendary sword Excalibur, wielded by the first Dark One, the sorceress Nimue.
After many centuries, as well as many Dark One's that followed Nimue, the previous Dark Ones Emma Swan and Killian Jones used the magic of the fully reformed Excaliber to absorb all the previous Dark Ones' powers, including their own, and in trying to destroy the darkness once and for all, Emma stabbed Killian while the darkness still resided in the sword, returning them both back to the way they were. However, they did know that the past Dark One Rumplestiltskin used the sword as a conduit to channel and move all the darkness into himself by using a potion he had on it, and in doing so, restoring status and his powers as the Dark One once again, only this time he was far more powerful, having absorbed the powers of every Dark One who has ever lived, including the original recipient of the power and immortality Nimue, Killian Jones (who's power came from that of Merlin's own powers), and Emma Swan (who is the Saviour and was told by Merlin to be the most powerful Dark One to date (until Rumplestiltskin's powers were restored), making him the most powerful sorcerer in all the realms, surpassing even Merlin.
The original color scheme of the dagger was a silver curved blade, etched with black writing and designs. However now, since Excaliber absorbing the power of ever Dark One who ever lived, the dagger is a black blade with silver writings and designs (the same designs with exact opposite color scheme).
- The importance of the knowing name on the dagger is an allusion on the original Rumpelstiltskin fairy tale, where he made a deal with the miller's daughter (revealed to Cora) to discover his name.
- Its existence marks a great importance to Hook as the only mentioned method of killing Rumplestiltskin, whom Hook desperately seeks to kill to avenge Milah. It is quite possibly the weapon he heard rumors of when "liberating" Belle.
- The dagger's design is based on ritualistic Indonesian knives.
- Cora refers to the dagger as a "kris dagger". A kris is a prized Indonesian weapon known for its distinctive, wavy blade. These blades are considered to be spiritual objects.
- If the Dark One is killed without the use of the dagger, he or she will expel the powers of the Dark One into the air after death without anyone else being next in line as the next Dark One, therefore ending the curse, however, they can be resurrected through the Vault of the Dark One.
- Belle is wearing a dagger-printed scarf while searching for the Snow Queen.
- Throughout the series, the written letters of the Dark One's name keeps switching from one side of the blade to the other. Sometimes, the letters are right side up when the dagger is held in the right hand and facing the wielder with the first letter at the tip of the blade. At other times, the writing is right side up when the dagger is facing the wielder and held in the left hand with the first letter next to the hilt.
- The new dagger has all the abilities of the old dagger, including the power to kill the Dark Ones, as well as control them, the only difference being the inverted color scheme, going from silver with black prints to black with silver prints. At other times, the writing is right side up when the dagger is facing the wielder and held in the left hand with the first letter next to the hilt.
- The blade of Excalibur, which the dagger was a part of, is normally silver-colored with black markings, but after the Darkness is absorbed into the ancient weapon, the blade becomes black, with silver markings, as once seen when the sword disintegrates in Emma Swan's hands.
- The new version of the dagger has the same color scheme, as does the Land Without Magic item called the "Holy Grail Twin".
- When King Arthur is talking to Sir David Nolan while sitting at the Round Table, a drawing of the Dark One's Dagger is lying on the table in front of him.
- In Isaac Heller's alternate reality, Emma Swan was wearing a pair of dagger-shaped earrings which is similar to the style of the Dark One's Dagger.