Heretics

Heretics are a group of Siphoner/Vampire hybrids originating from the Gemini Coven.

They were originally six witches, all of whom were exiled for their inability to perform magic unless it was siphoned by another source, such as a witch or a magical object. They were considered an abomination by the witches and were ultimately exiled sometime between the mid-late 19th century and early 20th century.

After being rejected by their coven, they were turned by Lillian Salvatore, a vampire and known Ripper, prior to the early 1900's and, as vampires, they were then able siphon the magic that gave them their immortality and vampire nature, making the Siphoners the only beings who could be both witch and vampire.

In 1903, Lily and her den returned to America, where they sought to find and destroy the Gemini Coven in revenge for exiling them. On the way there, Lily also turned a sick young man into a vampire, but abandoned him. However, the Geminis found them first and banished them to a prison world where they remained imprisoned for over a century.

History
The Heretics were once members of the Gemini Coven, a congregation of witches who had bound their entire individual magic to the successful transition from one leader to the next. The basis of this transition was a ritual known as the Merge: twins born to the line of the coven's leaders would perform the ritual on their twenty-second birthday, during which the strongest twin would survive and absorb the magical strength, as well as some personality traits, of the weaker one, killing the latter in the process. This allows them to ascend as the new leader of the coven and be reborn as a new being. As the spirit's of the twins merge into the winner's body, the winner contains aspects of each of the twins personalities, and gains access to enhanced magical powers, as the coven's leader binds both the magic and lives of the Gemini Coven's members into his or herself. As a result, if the leader dies before another leader emerges from a Merge Ceremony, all living members of the coven will die. This happened in I'm Thinking Of You All The While, when Kai killed Jo and her unborn twins to prevent the coven from having future heirs to strip him of his title as leader, and then committed suicide with Lily's vampire blood in his system to transition into a Heretic. As a result of Kai's death, all of the living members of the Gemini Coven died.

Unlike their fellow witches, the Heretics were unable to generate their own magic; instead they could only siphon it from other sources, such as other witches or enchanted objects. The Geminis considered these traits to be an abomination and denounced them as "heretics" before banishing them from the coven. The six Heretics made their way to Europe where they eventually met Lillian Salvatore at some point before 1903. A vampire Ripper, Lily had been massacring several thousand people in Europe in the years after she turned in 1858. After they had met, they all quickly became close friends, and Lily, she eventually turned them into vampires so they could remain together forever. However, due to a loophole of Nature due to their Siphoner identities, the Heretics found that they could, as vampires, siphon the magic that made them immortal as power source, and as such, they became both true vampire-witch hybrids, possibly even the first of their kind. As Siphoners, they can even cure themselves of the deadly werewolf venom (which would be fatal to any ordinary vampire who was unable to ingest Klaus's blood as a cure) by siphoning the magical basis of the werewolf venom.

Now in possession of great power, the Heretics returned to America with Lillian in autumn of 1903 to have their revenge on the Geminis. However, the coven got to them first in the New York Harbor and banished them to a prison world, a temporal time loop of the same day in 1903 (October 31/November 1) cast using the power of the Aurora Borealis overhead. Lily and the Heretics remained trapped in 1903 for over a century and eventually settled down in the prison world's version of Mystic Falls, Lily's hometown. Because the metaphysical rules of prison worlds exclude living beings as objects that are essentially copied into the prison world, the only blood which the Heretics could consume to survive was animal blood found in butcher shops and slaughterhouses, they were limited in their food sources; the era in which they were trapped meant that the transportation necessary to find more blood were scarce, forcing them to have to ration the small amount of blood they did have. The Heretics eventually refused to drink blood and gave Lily their rations, which forced her to control her blood lust by reducing her consumption so there would be enough blood to wake them in the case they were ever able to return to the living world. While Lily remained alive, the six Heretics suffered such intense starvation that they desiccated to the point of mummification, remaining that way for over a hundred years.

Season Six
In A Bird in a Gilded Cage, Lily's son Damon, his girlfriend Elena Gilbert and two witches, Bonnie Bennett and Malachai Parker (the current leader of the Gemini Coven), entered the prison world to free Lily. After explaining to her their situation, Lily agreed to leave with them at first, but then changed their mind when they realized that, by leaving Kai behind in punishment for his treatment of Bonnie, they didn't have enough magical power to bring the Heretics back with them. Lily fed one of the members of her "family" blood, trying to bring them along despite Damon's explanations, Damon made it clear that he would leave her behind, and Lily reluctantly left without them, though she made Damon promise they would come back for them later. Once they left, Kai, who was left behind by Bonnie, Damon, Elena, and Lily, later wandered into the Salvatore Estate to look for food, where he stumbled upon the one Heretic who was no longer desiccated, who ultimately was overcome by bloodlust and presumably fed on him.

In I Never Could Love Like That, Josette Laughlin, after learning that Kai had been trapped in the 1903 prison world, called her father, Joshua Parker (the former leader of the Gemini Coven), to inform him of Kai's imprisonment. Joshua then became worried and explained to Jo about the real reason why Lily and the Heretics were imprisoned before insisting that the Ascendant must be destroyed before Lily could bring back the Heretics. Jo later tells Elena and Damon the reason for the prison world's creation-- the Heretics, after being exiled by the Gemini Coven for being Siphoners, were turned into vampires by Lily and were subsequently turned into true vampire-witch hybrids as a result of their Siphoner nature. Jo told them to think of the Heretics as Kai with vampire bloodlust, and insisted that they were too dangerous to be freed into the world.

In I'd Leave My Happy Home, Kai was seen in the 1903 prison world making breakfast for himself and the Heretics. It also appeared that they had been feeding on Kai as well. When Kai rang the bell to call them to breakfast, he smiled and told them that they needed to eat and prepare themselves to finally leave the prison world and return home.

In I'll Wed You in the Golden Summertime, it was heavily suggested that Lily and Kai were working together, with the agreement that if Lily helped Kai with his plans, Kai would free the Heretics from the prison world, along with himself. When Kai and the Heretics escaped, he presumably freed Lily from her cell in the Salvatore boarding house and gave her the address for the warehouse where her family was being held. However, when Lily got there, she couldn't find them anywhere.

In I'm Thinking Of You All The While, it was revealed by Lily that, in exchange for freeing her family from the prison world so she could reunite with them, Lily fed Kai her blood as part of his plan. He then showed up at his twin sister Jo's wedding to Alaric Saltzman, killed Jo and her twin children to prevent them from stripping him of his coven leadership, and then killed himself in order to kill the entirety of the Gemini Coven, whose life forces and magic were all tied to his. Since he had Lily's blood in his system, he awakened in transition to become a Heretic vampire, and used his vampirism as a power source so he could telekinetically pull his father Joshua toward him and feed on him to complete his transition. When he was later bitten by the newly-triggered werewolf Tyler Lockwood, he revealed that Heretics can even overcome werewolf venom, which would be fatal to an average vampire, by siphoning the magic in the venom as a power source. However, Kai was ultimately killed by Damon Salvatore, who was furious at him for casting a stasis spell on Elena, linking her to Bonnie's life, and nearly killing Bonnie. Once Kai had died, the cloaking spell hiding the Heretics was broken, and Lily was finally able to reunite with her six Heretic family members. She also invited Enzo to join their family, and revealed that two of the Heretics are named Valerie and Malcolm.

Season Seven
In Wishing You Were Somehow Here Again, it is revealed in a flashback that Stefan talked with his mom, the leader of the Heretics. In present time, they kill two staff members of the Mystic Falls High School and a police officer when he wants to stop them.

In Why Do You Let Me Stay Here?, Nora and Mary Louise kill two girls from the local supermarket. Lily helps to remember all of them that they killed thousands of people and that they need to change their ways if they want to live in Mystic Falls. The Heretics are attacked by Caroline, Matt, and William. They survive and retaliate by attacking the Mystic Grill which kills a small number of people. They are stopped by Lily when she returns from visiting Enzo. Finally, a deal is made with the new Sheriff and the town is evacuated. They officially live in the Salvatore boarding house now. Malcolm is killed by Damon and Bonnie.

In A Date With Destiny, Nora and Mary Louise find Malcolm's body and bring it inside the boarding house. Lily is upset and abducts her son Stefan with the help of Beau. Enzo is ordered to kidnap Caroline, but Damon stops this from happening. He and Bonnie save Stefan while the remaining Heretics go to the Salvatore crypt to pay their last respects to Malcolm.

In A Nightmare on Sorority Row, the Heretics go to the House of Evil. Chaos ensues when Matt and Damon's seperate plans don't fully succeed.

In Something to Die For,

In The One That Got Away,

In Night of the Hunter,

In Wrapped in Red,

In A Long Way Down,

Trivia

 * According to Julie Plec, they are sociopaths who have killed thousands of people together.
 * The Heretics are the first group of witches who have been shown to keep their ability to practice magic after being turned into vampires as a result of their Siphoning ability.
 * As hybrids, they can draw upon their vampirism as an endless source of power.
 * It is possible that the Heretics are the only group of true vampire-witch hybrids in existence.