Klaus Mikaelson's (Joseph Morgan) hybrid curse, eventually broken during the series, plays a central role in the events of The Vampire Diaries. Klaus is a member of the Original family. In the Middle Ages, when werewolves begin terrorizing the village the Mikaelsons live in, Klaus's mother Esther Mikaelson (Natalie Dreyfuss)—a witch—casts a dark magic spell that turns Klaus and his siblings into the first vampires. Unlike the rest of his family, Klaus is also placed under a second spell called the hybrid curse. This curse drives his character arc throughout The Vampire Diaries and The Originals.
In The Vampire Diaries, season 2, episode 8, "Rose," Klaus is first introduced as a villain of the series. Klaus is determined to break his hybrid curse at any cost, harming his family and others in the process. Breaking the hybrid curse requires a human doppelgänger. When doppelgänger Katherine Pierce (Nina Dobrev) ruins his first attempt at breaking the curse, Klaus slaughters her whole family and chases Katherine around the globe to torture and kill her. About half a millennium later, Klaus hears of another human doppelgänger named Elena Gilbert (Nina Dobrev). He makes his way to Mystic Falls, ready to use her to break the hybrid curse.
The Hybrid Curse Was Placed On Klaus To Prevent Him From Becoming A True Hybrid
As a young adult, the hybrid curse is placed on Klaus by his mother Esther. Klaus's mother Esther had an affair with the village's werewolf alpha, conceiving Klaus. In The Vampire Diaries lore, any character born with the werewolf gene begins turning at every full moon once they kill, whether intentionally or accidentally, for the first time. When Esther turns her family into vampires, their bloodlust makes killing inevitable. Upon killing for the first time, Klaus's werewolf side and Esther's affair are revealed. Esther's husband Mikael (Sebastian Roché) flies into a rage and murders Klaus's biological father. He also begins abusing Klaus more, calling Klaus an abomination, and referring to him as "the bastard" instead of by his name.
Esther decides to bind Klaus's werewolf side with the hybrid curse for two primary reasons. Firstly, she believes this will prevent further wrath from Mikael. Secondly, she believes Klaus will be too powerful as a true hybrid. The hybrid curse physically limits Klaus's abilities. With the curse in play, Klaus cannot turn into a werewolf. He doesn't possess the unique traits of a true hybrid. The hybrid curse strips him of transitioning at any will, day walking, using his werewolf bite, curing the werewolf bite, and near invincibility. Klaus also possesses the basic strengths of a vampire, but they aren't amplified in the way they are after breaking the hybrid curse. Additionally, he cannot make any more hybrids.
Klaus's Hybrid Curse Was Eventually Lifted Through the Sacrifice Ritual
In order to break the hybrid curse in The Vampire Diaries, Klaus needs to go through a sacrifice ritual. The sacrifice ritual requires the following ingredients: the moonstone that bound the hybrid curse, a full moon, one vampire, one werewolf, a human doppelgänger, and a witch to cast the spell. The ritual must occur at the birthplace of the doppelgänger. In the case of Elena, this means Mystic Falls, Virginia. Klaus only fully plans two attempts at breaking the hybrid curse—one in 1492 and one in 2010.
Eventually, Klaus's hybrid curse breaks in The Vampire Diaries, season 2, episode 21, "The Sun Also Rises," with the help of a witch named Greta Martin (Lisa Tucker). During the ritual, he sacrifices a werewolf named Jules (Michaela McManus). He also sacrifices Elena's aunt Jenna (Sara Canning), who begins transitioning into a vampire only one episode earlier. Stefan Salvatore (Paul Wesley) and Damon Salvatore (Ian Somerhalder) offer themselves up in her place, but Klaus enjoys the symmetry of the sacrifice of three women. Klaus then proceeds to drink Elena's blood to the point of her death, though she survives due to a trade of life with her uncle and biological father John Gilbert (David Anders). Elena's best friend Bonnie Bennett (Kat Graham) tries to take down Klaus both before and after the ritual, but she doesn't succeed.
The breaking of the hybrid curse has many ripple effects on the characters and plot of The Vampire Diaries. After breaking the curse, Klaus also starts making hybrids. Tyler Lockwood (Michael Trevino) becomes Klaus's first successful hybrid, causing a rift between him and Caroline Forbes (Caroline King). Caroline grows into a stronger, more outspoken character without Tyler. Additionally, when Klaus's hybrid blood becomes the only cure for a werewolf bite, characters start making deals with Klaus in exchange for his blood. Most notably, Stefan agrees to leave Mystic Falls and work with Klaus in exchange for curing his brother's werewolf bite. Stefan's absence allows Elena to develop deeper feelings for Damon. Ultimately, Klaus breaking the hybrid curse in The Vampire Diaries pushes many characters and relationships in a new direction that allows growth.