The Curse of the Sun and the Moon was bound with Petrova blood. And the Petrova doppelganger was created to undo the spell. Thus, Klaus needed Katherine to drain her blood as a sacrifice to break the spell.
Katherine was the next doppelgänger after Tatia; a woman whom Klaus was in love with and who was identical to Katherine. Katherine "was taken" with Klaus at first when she had been human. Katherine had been the object of Klaus' hatred for over five hundred years after she turned herself to evade his plans.
Throughout The Vampire Diaries
The Original Hybrid, Klaus Mikaelson, and the second-known Petrova Doppelgänger and newly-turned human/witch of the Traveler sub-culture, Katherine Pierce, slept together in Graduation and conceived their daughter, Adyelya.
Firstly, it was because Esther used Tatia's doppelgänger blood to make Klaus's Werewolf side dormant, combined with the help of the Full Moon and the Moonstone. To break the curse, Klaus needs Elena's blood to unlock his Werewolf side yet again which requires him to drain the blood of the female doppelgänger.
The real curse was the hybrid curse: a curse placed on Klaus, an original vampire, to keep his werewolf side dormant. To break the curse, Klaus needed to sacrifice a werewolf, a vampire, and a doppelgänger, aka Elena.
The final act of the sacrifice was to feed on the doppelgänger to her death. Klaus followed through with the sacrifice and fed on Elena's blood until she died and successfully broke his curse.
Matt eventually contacted his dead sister, Vicki, who told him that the fact that Elena was still alive was the reason why Klaus couldn't make more hybrids.
He was able to cure werewolf bites because he was an Original Hybrid (an Original Vampire who's also an active werewolf). 2)Klaus is the only werewolf-vampire hybrid who activated his wolf gene after becoming vampire (rest of hybrid were Werewolves before they become Vampires).
Loyalty. Ansel and Niklaus Mikaelson's Family is an unknown ancient bloodline of very powerful werewolves that goes back at least a thousand years. They are all members of the North East Atlantic Pack, that is native to America and is said to have existed since the genesis of the werewolf species.
He was forced to send her away to live with her aunt Rebekah to protect her from those who would try to harm her, or those who would target her for being a Mikaelson.
In the season four finale of The Vampire Diaries, Graduation, Katherine slept with Niklaus Mikaelson and conceived her daughter, Adyelya.
Also, in the books, Elena and Katherine were half sisters due to having the same mother (though Katherine suspected Elena was a possible descendant), but in the TV series, they are related by blood and Elena is her descendant.
The Curse of the Sun and the Moon was bound with Petrova blood. And the Petrova doppelganger was created to undo the spell. Thus, Klaus needed Katherine to drain her blood as a sacrifice to break the spell.
Tatia is the first-known Petrova doppelganger, and her blood was used by Klaus' mother, Esther, to turn her children and husband, Mikaelson, into the first vampires. Following suit is Katherine Pierce, and the modern-day heroine, Elena Gilbert.
Katherine was the next doppelgänger after Tatia; a woman whom Klaus was in love with and who was identical to Katherine. Elijah also had feelings for Tatia as well (she was his first love), and she chose Elijah over Klaus.
As a tough and self-centered character, Katherine does what she wants and doesn't appear to be afraid of anyone or anything. However, she has been frightened of Klaus for a long time, which emphasizes their tense and difficult relationship.
The Primordials are the most powerful werewolves in existence and no werewolf can match their power, they possess some abilities that normal werewolves don't. Their abilities are at their peak during a full moon. Heightened Senses - Primordials have extremely enhanced, keen senses of hearing, sight, smell, and taste.
Jerick. Despite appearing in just two episodes of The Originals, Jerick (Kevin Savage) reigns supreme as one of the few Evolved Alpha Werewolves in the notorious Crescent Wolf Pack.
Julie Plec confirmed that Klaus is the origin of the bloodline Stefan, Damon, Caroline and Elena are from. It is possible to unlink a vampire from their bloodline using magic. This was successfully performed on Klaus by Davina.
As I mentioned, Elena is the only female werewolf in Armstrong's supernatural world. This is because the gene that creates a hereditary werewolf is on the Y chromosome, so it's only inherited by males.
A vampire's death by a werewolf bite is very slow and painful, which is why many who are bitten are ultimately "mercy-killed" before they reach that point. Werewolf bites cannot kill an Original Vampire, as Elijah Mikaelson mentioned to Elena Gilbert when they were talking about how to kill Klaus Mikaelson.
Most of the second season of The Vampire Diaries, The Originals' parent show, saw Klaus trying to unlock his werewolf side with a ritual, but while he finally managed to do so, fans of both series never actually saw Klaus transition on screen.
An Original Immortal Tribrid is a rare species that was formed in 2027 by the death of Dianna Mikaelssen, who is the only known member of this species. An original immortal tribrid is a combination of an original vampire, a werewolf, and a witch.
Later, the original family, Klaus and his siblings were added to the plot. They required a daylight ring, just as other vampires, to exist in the sunlight.
Hayley Wasn't Sired To Hope
At the end of the first season of The Originals, Hayley dies with Hope's blood in her system, and thus she transitions into a hybrid after completing the transition with more of Hope's blood.