|“||Infuriated by this betrayal, my father forced our mother to cast a spell that would suppress Klaus' werewolf side, denying him any connection with his true self.||”|
She placed the spell on Klaus shortly after he was turned into a vampire, when he made his first human kill and triggered his werewolf abilities. This event led to the revelation that Klaus was the product of an affair Esther had with a werewolf villager, Ansel, and was the world's first vampire-werewolf hybrid. This curse suppressed Klaus' werewolf side and kept him from being able to access any of the powers that came from being a true hybrid.
The Hybrid Curse
In Klaus, Elijah revealed to Elena that The Sun and the Moon Curse was actually a work of fiction that Klaus and Elijah had made up many centuries prior, and that the real curse was placed on Klaus himself over a 1,000 years ago. Niklaus, along with his other siblings and parents began their lives as humans. Shortly after Esther cast the spell to turn Klaus, his siblings (Elijah, Finn, Kol, and Rebekah), and his father Mikael into Original vampires, it was revealed that Niklaus was not his father's son.
His mother had previously been unfaithful with a werewolf chief in her village named Ansel, which resulted in Klaus' conception and werewolf heritage. Therefore, Niklaus was not only a vampire, but he was also a werewolf—a true hybrid and the first of his kind. Such a being was deemed too powerful to exist, and as a result of this imbalance of nature, compounded by Mikael's rage regarding Esther's affair with Ansel, Esther felt obligated to curse Niklaus to keep his werewolf side from manifesting and to try to repair the imbalance she had caused.
Because all magical spells require a loophole, there was one ritual that could be performed in order to break Klaus' curse, which involved channeling the power of a full moon into the moonstone that bound the original curse. Then, a werewolf and a vampire must each be sacrificed by Klaus to represent both sides of his heritage before he would finally drink the blood of a human doppelgänger in the birthplace of the doppelgänger in question to the point of their death. Klaus originally intended to break his curse in 1492, when Klaus finally found the human doppelgänger he needed in order to break the curse, Katerina Petrova, who had just moved to his current residence in England after being exiled from Bulgaria. Unfortunately for Klaus, Katerina was somehow informed of his plan, and eventually tricked Rose into turning her into a vampire so that her blood would ultimately be useless in the ritual.
Though the curse was supposed to stop him from unleashing his full potential as a werewolf-vampire hybrid, it seemed to only prevent him from voluntarily accessing it. He was still offered some of the protections of being a hybrid. For example, he still possessed the resistance to injuries caused by silver, which resulted in the white oak ash daggers not working on him as they did his siblings. His blood also still acted as the only known cure for a vampire who has been bitten by a werewolf or hybrid. The curse did, however, succeed in preventing Klaus from using his blood to sire a new species of hybrids.
In What, Will, I, Have, Left, the unnamed Witch that was working with Greta Sienna placed the curse on Hayley Marshall-Kenner. By binding her werewolf nature, she was turned into a vampire. The Witch demonstrated this by moving Hayley's hand into the sunlight, burning her skin without a daylight ring. Hayley took advantage of the situation to attack and kill the unnamed witch, while Hope temporarily broke Roman's neck. Not long thereafter, Hayley sacrificed herself to kill Greta and save Hope.
Breaking the Curse
The ingredients for the breaking of the curse are the moonstone, a witch to do the spell, and a werewolf, a vampire, and a doppelgänger to sacrifice. In Niklaus' second attempt at breaking the curse, he used Elena as the doppelgänger, Jules as the werewolf, Jenna as the vampire, and Greta Martin as the witch to perform the ritual, which was done at Steven's Quarry. First, the witch channeled the power of the full moon to release the spell which was bound within the moonstone. Then, Niklaus being both werewolf and vampire, sacrificed one of each; first Jules, the werewolf, followed by Jenna, the newly-turned vampire. Finally, in the last step of the ritual, Niklaus drank the blood of the Petrova doppelgänger, Elena, until the point of her death. Klaus killed them each in the most common ways to kill their specific species: heart extraction (werewolf), a stake through the heart (vampire), and draining their blood (human doppelgänger). Elena willingly chose to be sacrificed in order to prevent any of her loved ones to be hurt in the process. Stefan tried to give himself up to take Jenna's place as the sacrificed vampire, but Klaus refused, claiming he liked the poetry of sacrificing "three goddesses" at an altar.
The Sun and the Moon Curse
The Sun and Moon Curse was said to be an ancient Aztec curse that a great shaman cast 1000 years ago on vampires and werewolves. The curse decreed that the werewolves would only be able to turn into a werewolf on a full moon, and that the vampires would be weakened by the sun, which made the werewolves "servants of the moon" and the vampires "slaves to the sun." Since the curse was fabricated by Niklaus, it could be assumed that werewolves have never been able to control their transformation. The Sun and Moon curse was first mentioned in Bad Moon Rising by Vanessa Monroe, who explained it to Elena, Damon and Alaric.
Rose revealed that the blood of a human Petrova doppelgänger, ergo Elena, was needed to break the curse. Rose also explained that the Originals, the oldest and most powerful of vampires, would come after Elena in order to break the curse. In Katerina, it was revealed that Niklaus actually wanted to break the curse himself. He first planned to sacrifice Katherine in the 15th century, as evidenced by the flashbacks in the episode, but she escaped and became a vampire before he was able to finish the ritual. Slater further elaborated the supposed after-effects of breaking the curse: if vampires were to break the curse, they would be able to safely roam during the day, leaving werewolves able to turn only on a full moon forever. But, if the werewolves were to be the ones who broke the curse, then they would be able to turn whenever they wanted, while vampires were left to only venture outdoors at night, and would be unable to do anything about it, aside from wearing daylight rings. In short, only one side of the curse could be broken, making the other side permanent. Elijah revealed to Elena that the sole reason why he and Klaus fabricated all of these various myths of "The Sun and Moon Curse" in so many various different cultures was to ensure that the involved species (werewolves and vampires) would be on the look out for a long-lost moonstone and doppelgänger over the centuries was by making their discovery relevant to those species.
A witch was needed to cast a spell to break the curse. A vampire and a werewolf were both needed to be sacrificed in order to have their blood spilled over the moonstone to release the curse from the stone. Once the sacrifice of the doppelgänger was made, Niklaus would complete his first transformation into a werewolf/hybrid since he originally triggered his curse over a millennia earlier; as the werewolf transformations during the full moon are so painful, the agony of it would briefly leave Klaus more vulnerable than he had ever been since becoming a vampire.
The curse must be broken in the birthplace of the doppelgänger during a full moon. It was Katherine's original plan to come back to Mystic Falls and barter the ingredients of the spell in exchange for her freedom. Her plan was to hand over Lucy, Bonnie's cousin, as the witch to perform the spell, Mason Lockwood as the werewolf, Caroline Forbes as the vampire, and Elena as the doppelgänger as a way to help Klaus break the curse. Since Lucy betrayed Katherine and Mason was killed, Katherine turned to her back-up plan, which was to use Bonnie Bennett to replace Lucy as the witch and Tyler Lockwood to replace his uncle Mason as the werewolf sacrifice.
After Klaus succeeded in undoing the hybrid curse, he set out to propagate his new species by siring werewolves with his blood, which turned them into hybrids like himself. In a way, the Sun and Moon Curse proved to be partially true after all, as his hybrids were undead immortals capable of walking the day without needing daylight rings, and at the same time, they were also werewolves who could transform outside of the moon's influence (or, conversely, choose not to turn at all.)
However, Esther believed she had put a fail-safe into the curse, as the ritual to break it involved killing the doppelgänger; since doppelgänger blood was also necessary to successfully sire hybrids, and since at the time, it was believed the Petrova line was the only existing bloodline of doppelgängers, Esther believed that even if Klaus managed to break the curse, he would at least be unable to succeed at creating his new all-powerful species. However, out of desperation to ensure Elena didn't die in the sacrifice, her biological father John magically gave up his life in order to allow Elena to be resurrected afterward, and this act ultimately allowed Klaus' dream of creating a hybrid army to come to fruition.
In Katerina and Klaus, it's revealed that in 1492, Elijah and Niklaus obtained all the necessary elements to break the curse, but before they could perform it, Katherine escaped with the help of Trevor. He sent her to Rose's home, thinking Rose would help him, but instead, she insisted she would hand her back over to Klaus as soon as night fell and she could go outside. Before she was able to do so, however, Katherine managed to trick Rose into feeding her vampire blood by stabbing herself in the stomach. Then, when Rose left the room for a moment, she hanged herself with rope, turning herself into a vampire and making herself unable to serve as the doppelgänger in the ritual.
In the 21st century, Katherine wanted to obtain her freedom after she had spent the last 500 years on the run from Klaus after she betrayed him and he slaughtered her entire family in Bulgaria. In her efforts to gain pardon for her crime against him, she tracked down all the ingredients he needed to break the curse, with the intention of trading them in exchange for her life.
Ingredients for the curse:
- 10th century: Klaus (hybrid), Esther (witch), Tatia (doppelgänger). Sacrifice of a werewolf or vampire not necessary to cast the curse, as the Original vampires were the only vampires to exist at the time.
Klaus' attempts to break the curse:
- 1492: Unnamed vampire, Unnamed werewolf, Unnamed witch, Katerina Petrova (doppelgänger).
- 21st century
- Katherine's victims (Plan A): Caroline Forbes (vampire), Mason Lockwood (werewolf), Lucy (witch), Elena Gilbert (doppelgänger).
- Katherine's victims (Plan B): Caroline Forbes (vampire), Tyler Lockwood (werewolf), Bonnie Bennett (witch), Elena Gilbert (doppelgänger).
- Niklaus' victims (Decoy/Initial): Caroline Forbes (vampire), Tyler Lockwood (werewolf), Maddox (witch), Elena Gilbert (doppelgänger).
- Niklaus' victims (Backup): Jenna Sommers (vampire), Jules (werewolf), Greta Martin (witch), Elena Gilbert (doppelgänger).
- Damon Salvatore: The first choice for Klaus' back-up vampire in the 21st century was Damon. He initially offered himself up as a last-ditch effort to take Jenna's place in the sacrifice. However, Klaus realized he had been bitten by a werewolf and thus his blood was unfit to be used in the sacrifice.
- Stefan Salvatore: The second choice for Klaus' back-up vampire was Stefan. He, too, offered himself up to Klaus to take Jenna's place in the ritual, but Klaus had other unknown plans for him, so Klaus spared Stefan's life and forced him to watch as he killed Jules, Jenna and Elena in turn.
- The Hybrid Curse was cast on Niklaus, and later on Hayley. With Klaus's curse being broken and Hayley dying while under the curse's effect, no active form of the curse exists.
- When Esther cast the Hybrid Curse over 1,000 year ago, she used the blood of Tatia, a lover of both Elijah and Klaus, who was also one of Amara's doppelgängers. It can be assumed that Esther knew of Tatia's status as a doppelgänger of Amara
- In Red Door, it was confirmed that Tatia's blood was used in both the spell that created the Original vampires as well as Klaus' Hybrid Curse. She willingly gave her blood to Esther for the first spell, while the blood used for the Hybrid Curse was taken from her body after Elijah accidentally killed her from feeding on her to death.
- Despite being unable to access his werewolf abilities, Klaus still possessed the immunity to wounds made by silver that all werewolves have. This meant that he couldn't be daggered like his step-father and siblings. Whether this was a magical oversight on Esther's part, a result of Klaus' hybrid nature being rapidly activated and then suppressed by the curse, or just an inherent trait of the werewolf gene that cannot be suppressed has yet to be explained.