The White Oak Ash Daggers are weapons that were forged by witches as a means of subduing an Original Vampire. If dipped in the ash of the White Oak Tree dating from the genesis of the vampire race and placed in the heart of any Original Vampire, they'll remain dead for as long as the dagger stays in.
The daggers must be used only by humans because if wielded by a monster - refers to normal vampires and possibly werewolves - the dagger will claim both lives.
The daggers are known to have existed as early as 900 years before Pilot, back in the twelfth century. The original known owners were the members of The Brotherhood of The Five. It is unknown if the witch who enchanted the daggers was the same witch of The Five, or someone else, but the former is most likely.
Alexander showed Rebekah his dagger and explained that it was a weapon to use against a special kind of vampire (referring to an Original Vampire) and that it would serve as a useful weapon until they could find the cure and permanently kill the Originals. He then plunged it into Rebekah's heart. On the same night the others tried to disable the Originals, daggering all of Rebekah's siblings. However, due to Klaus' dormant werewolf side and the fact that the daggers were made of silver, the dagger didn't work on Klaus. Their plan backfired when the hybrid killed them all in retaliation.
Klaus revived all of his siblings but Finn, and kept the daggers for whenever he wished to impose his will over his siblings. He would use the daggers when in disagreement with his siblings, such as when he daggered Kol in the early 1900's and Rebekah in the 1920's. Keeping the daggers in their hearts kept them neutralized so Klaus kept his daggered siblings wherever he went, resting in coffins.
One of the daggers ended up in the possession of John Gilbert, who tried to have Damon Salvatore use it on the Original, Elijah, knowing that if a vampire tried to use it on an Original, they'd die from using it. This was discovered though and Alaric Saltzman stabbed Elijah with it instead. However, he was unaware that removing the dagger from the body would revive the Original, Elijah waking shortly afterward. He was stabbed once more, this time by Elena Gilbert, who left the dagger in his body this time. The Salvatores kept Elijah's neutralized body in their cellar but when Elena felt he was the only one who could take on Klaus, she removed the dagger from his body and gave it to him as a peace offering, showing him she did not intend to threaten him again. Klaus broke the curse that prevented him from becoming a hybrid and once he did, daggered Elijah and stored him in a coffin with the rest of his siblings.
Klaus removed the dagger from his sister once he needed her to figure out why he couldn't make successful hybrids. Upon awakening, she stabbed him with the dagger, knowing it wouldn't kill him but hoping it would hurt him, which it did. When Mikael came wanting to hunt down Klaus with a white oak stake he had, the only weapon that could completely destroy an Original, he had Elena dagger him as part of the plan to lure Klaus to him. He was then undaggered once they had set their trap. Shortly after, Elena used the dagger on Rebekah, neutralizing her once more, fearing that she would betray them and aid Klaus. Rebekah was kept in the Salvatore cellar like her older brother, Elijah, had been. When her body was returned to Klaus, he removed the dagger but had then learned that Rebekah was now aware of the fact that Klaus had killed their mother, pushing the dagger into her once she started to awaken, fearing her reaction. The coffins of the neutralized Originals were stolen and after a struggle, returned to Klaus, but Damon had pulled the dagger from Elijah's chest when he returned him, reviving Elijah. Elijah removed the daggers from the rest of his siblings who were far from happy with Klaus. Finn and Rebekah stabbed him with the daggers, knowing they wouldn't neutralize him but would hurt.
After the Originals were linked as one, the Salvatores gave the dagger they had possession of to Alaric Saltzman, who daggered Kol, which neutralized all the Originals but Klaus, who quickly found them and removed the dagger from his younger brother. After Esther had possessed Rebekah, she had Alaric dagger her daughter while she returned to her original body. Once she died, Klaus removed the dagger from Rebekah.
When Klaus thought Rebekah knew too much about the cure, risking their search for it, he daggered her once more. Some time later, her neutralized body was found by April Young who removed the dagger. When Kol discovered the truth about the cure and the ancient immortal Silas, he tried to stop the hunt for the cure and Rebekah threatened to dagger him, which upset him greatly, in disbelief that she would resort to daggering siblings like Klaus did. Kol stole the rest of the daggers from Klaus, with only Rebekah remaining in possession of one, which she eventually gave to Stefan.
- The dagger has to be dipped in the ash of the white oak tree. Then a human has to plunge it into an Original vampire's heart. This will kill the original for as long as it remains in the heart.
- It is unknown if or not the dagger has to be dipped every time, before it is wielded. Klaus, once, in a fit of rage, pulled out Kol's dagger to kill Elijah but didn't go through with it. It probably wouldn't have worked.
- It is not supposed to be wielded by demons as it would claim both lives - of the Original as well as the demon. It is not yet known what falls under the definition of demon, but normal vampires are known to be included (the word "demon" probably refers only to vampires as the original owners, the five, have only referred to them as demon).
- A definitive scene depicting what exactly happens to vampires, when they use it, has not yet been shown. However, it is possible that some vampire must have attempted to use it, which is why the writings suggest that demons mustn't use them.
- Apparently, Original Vampires themselves don't seem to be under the definition of 'demon'. Still, Klaus has been the only one who has ever daggered his siblings on screen. It might not affect Klaus for three reasons:
- Original vampires were made to protect themselves; nature didn't consider them as demons. Rebekah attempted using the dagger on Kol but she didn't go through with it, so it is unknown what happens to originals other than Klaus when they use it.
- Klaus has a werewolf side, so silver based magic never worked on Klaus. Even if other Originals could get neutralised along with the Original they daggered, the same wouldn't affect Klaus. This by extension also means that werewolves can safely dagger an Original.
- The Spell that created the Originals decreed that only wood from the White Oak Tree can kill them. From this perspective, it would explain why Klaus can safely dagger another Original.
- It is unknown what happens if the daggers are used on normal vampires.
- It is possible that nothing happens, and they just heal once the daggers are removed, as the spell that enchanted the daggers is only known to be meant for originals.
- However, all weaknesses of originals have been generalised in normal vampires (any wood kills vampires as opposed to Originals dying only off of white oak wood). Wood just temporarily disables originals. So, it is possible that the daggers would kill a vampire permanently.
- Elijah (neutralized by Alaric Saltzman, Elena Gilbert and Klaus and The Five)
- Rebekah (neutralized by Alexander, Klaus, Elena Gilbert and Alaric Saltzman)
- Mikael (neutralized by Elena)
- Kol (neutralized by The Five, Klaus and Alaric Saltzman)
- Finn (neutralized by The Five and Alaric Saltzman)
- The daggers strongly resemble the seven "holy daggers" that were featured as the means to kill the Antichrist, Damien, in The Omen film series.
- Stefan now has possession of the last known dagger which was given to him by Rebekah, the rest of the set being stolen by Kol and in an unknown location.
- In A view to kill, Rebekah wanted to dagger Kol, but that would have led to her demise as well.
- A reason why Klaus never died from daggering his siblings was because of his werewolf lineage.