Linking spells can vary with the most extreme of cases, be used by a Coven to share power at the cost of also sharing their life-force such that linked beings who are can all potentially die if just one of the linked were to be killed in some way.
Doppelgängers, being modeled after immortals and other doppelgängers, have an inherently strong mystical connection to each other that can be used to invoke linking spells using the mystical properties of their blood.
Throughout The Vampire Diaries Series
In Masquerade, Katherine was intent on getting the moonstone back from Stefan and Damon, to the point where she created an entire scheme to get it back. To do this, she enlisted the help of a witch named Lucy, who apparently owed Katherine a favor; she linked Elena and Katherine together on so that Stefan and Damon would not be able to hurt or kill her without doing the same to Elena. Not knowing this fact, the brothers tried to fight her and kill her until Jeremy, who saw Elena getting repeatedly injured in the same areas and in the same manner that Katherine was being injured by the Salvatore brothers, ran to the room where the three were fighting and told them what had happened. Afterward, Lucy, who had obtained the moonstone from Bonnie, finally handed the stone over to Katherine, on the condition that her debt to Katherine was paid after this favor. However, unbeknownst to Katherine, Lucy had unlinked her from Elena and cursed the moonstone so that it would incapacitate her upon touching it.
In The Sun Also Rises, Bonnie linked Elena and John, so John died when Elena woke up in transition after the sacrifice, in which she was killed by Klaus with vampire blood in her system; and his transferred life-force allowed her to remain a human.
In Dangerous Liaisons, Esther had just returned from the Other Side and had led her children (the Originals vampires Finn, Elijah, Niklaus, Kol, and Rebekah Mikaelson) into believing that she has returned with the sole intention of reuniting as a family again. However, it soon became clear that evening at the ball held by the Mikaelson family that she had other motives, as she had invited Elena to the ball under the guise of getting to know her better. She instructed Finn to lead Elena upstairs to the study so that they could speak privately while the party was underway. When they finally met for the first time in person, Esther informed Elena that she would answer her questions, including her plans for her children. She confessed to Elena that she planned to link her children together in order to finally kill them, but added that she would need Elena's doppelgänger blood in order to cast the spell, and after some consideration, Elena agreed to give it to her so she could move forward with her plans.
That night, after all of the Mikaelson siblings had ingested the champagne laced with Elena's enchanted blood, Esther and Finn completed the spell so that they could all finally be bound together. Finn, who was acting as the "sacrificial lamb" for Esther's scheme, cut his hand to use his blood in the spell. Upon dripping his blood onto a sheet of parchment with the names of all five Mikaelson children written in Runic, Esther chanted the incantation, which resulted in the blood magically forming a tree on the parchment and sealing the spell, linking all five Mikaelsons as one.
Her plan after this was to perform a spell to reverse her Finn's vampirism, making him (and, through the link, the rest of his siblings) human again. Once they were mortal again, Esther planned to then kill a willing Finn, which would kill them all. Unfortunately for her, Elijah forced Elena to tell him what she had planned, and afterward, Klaus forced Bonnie Bennett to undo the linking spell by threatening to kill her friend Jeremy.
In After School Special, in an attempt to cast a protection spell on Atticus Shane using Expression magic, Bonnie accidentally linked him to April Young, resulting in both of them displaying the same effects of being drowned after Kol forced Shane's head underwater. However, though Shane recovered on his own because of the spell, April continued to cough up water and collapsed until Bonnie was able to magically undo the damage.
In Because the Night, Aja, in an attempt to rid Bonnie of her addiction to Expression magic, asked the eleven other witches in her coven to link themselves to her so they could exponentially amplify her power and give her the boost she needed to overpower Bonnie. However, when it became clear to Aja that Bonnie was under Silas' control, she determined Bonnie to be a lost cause and prepared to kill her. Caroline, not wanting her best friend to die, impulsively stabbed Aja in the stomach to keep her from stabbing Bonnie, which ultimately killed the rest of the coven along with her. This unintended massacre of twelve witches fulfilled Silas' goal to complete the Expression Triangle and made him one step closer to fulfill his own agenda.
In The Walking Dead, Bonnie linked herself to Katherine so that she could ensure that Katherine wouldn't come into contact with Silas, as he would be able to use his psychic abilities to read her mind and know that Bonnie was planning to use the Expression Triangle to lower the veil separating the living world from the Other Side so she could contact Qetsiyah and gain her help in vanquishing Silas. That night, a grieving Elena attempted to kill Katherine, and because she was still linked to Bonnie, she nearly killed Bonnie as well. Fortunately for them both, Stefan intervened at the last moment and saved both their lives.
Throughout The Originals Series
In Always and Forever, Jane-Anne Deveraux linked the pregnant Hayley Marshall to her sister Sophie as a way to maintain leverage over Klaus and force him and Elijah to help them overthrow the leader of the supernatural community of New Orleans, Marcel Gerard. However, in Fruit of the Poisoned Tree, this linking spell was broken by Davina Claire, who used a spell from Esther's grimoire that was given to her by Elijah.
In Rebirth, Davina revealed to Mikael that she was planning to use the opposite of a linking spell in order to sever the link between Klaus and two of the vampires he sired, her good friends Marcel and Joshua Rosza, whom she didn't want to die when she finally allowed Mikael to kill Klaus. However, in Red Door, Kol (under the guise of a witch named Kaleb), destroyed the Kandahar root Davina planned on using to bind her unlinking spell, destroying her chance of succeeding in breaking Marcel and Josh's links to Klaus' sire line.
In A Streetcar Named Desire, TBA
In The Bloody Crown, TBA
- Esther Mikaelson used a linking spell when performing the immortality spell, according to Kol, this was to add protection to her family.
- The linking spell linked the Mikaelsons and their sireline together, so that if one died, they all died.