Sire Bond

A Sire Bond is the link between the Sire and their Sired that exists within the vampire and hybrid species. The Sired complies with the direct and indirect orders of the Sire in order to show its loyalty. It is mentioned for the first time in Smells Like Teen Spirit, when Damon reveals Tyler is sired to Klaus, and while he says that it is very rare in vampires, it is apparently not so rare in hybrids.
== Hybrids ==
: "He's been sired. [...] He feels loyal to Klaus because Klaus's blood created him. [...] He'll seek acceptance from his master. It's really rare. But maybe not so much in hybrids."
: —Damon in Smells Like Teen Spirit
Hybrids are loyal to their sire, and will seek their acceptance because their blood created them - this is called a "Sire Bond". Every hybrid is very loyal even when acting out of free will they can't stop doing what their sire says. Presumably, this also causes them to seek them out whenever their presence may be required.
In Homecoming, Tyler gets everyone to go to his house for the homecoming dance after the gym gets flooded. It is implied later on that he had something to do with it. It is revealed that he did all of this for Klaus, and that the party is not a homecoming dance so much as a wake for Mikael. When Klaus goes to Tyler Lockwood's he shows him the other Hybrids he has created, like Tonyhis right hand man and Mindy. Klaus says that they will protect him no matter who gets in the way. When Klaus talks to Katherine (posing as Elena) that if he dies, he's made sure his hybrids will kill Damon.
Tony is later inside The Lockwood Mansion with Klaus when Mindy said that Klaus has a visitor which she revealed to be Mikael, Klaus`s step-father, Klaus said to Tony that he knew what to do. Tony then said to the other hybrids to come with him and wait for Klaus to give the order to attack Mikael and anyone else who is with him. Tony and the other Hybrids tried to save Klaus when Damon attacked him, but Katherine threw grenades (containing vervain and/or wolfsbane) at them, incapacitating them.
Meanwhile, Tyler injects Caroline with vervain, knocking her unconscious. When she wakes up he asks her what is going on. She says that she doesn't know but if she did, she wouldn't tell him due to his sire bond with Klaus. Caroline asks him why he willingly follows Klaus, as he doesn't have any free will when he does. Tyler responds that he never had free will, the full moon commanded him before, and Klaus saved him from that. The conversation ends with both parties angry at the other and the relationship in doubt.
In The New Deal, during the days following Stefan Salvatore's theft of his family, Klaus searched for him. When this proved to be a failure, he decided to convince Elena and Damon to help him find them instead. Having Tyler befriend Jeremy in order to get him to be vervain-free, Klaus was able to compel Jeremy to stand in the street while Tony ran him down in an SUV.
In Our Town, Tyler was ordered to bite Caroline but he refuses. While Elena, Bonnie, Matt and Caroline were at the cemetery celebrating Caroline's 18th birthday, Tyler comes later. In the woods he talks with Caroline and accidentally bites her. He then goes to Klaus, who healed her. Tyler called Bill Forbes to return to Mystic Falls, hoping he could help him fight off his sire bond to Klaus. He chains Tyler up and orders him to turn at will. Tyler refuses but then he tries to change. Because it hurts too much and he didn't want to continue, Bill threatens that he would decapitate Tyler right now if he didn't keep on. Later, Tyler lost control, broke out and attacked Bill, almost killing him. Tyler showed up at the hospital and apologizes. Bill told him that he has to turn every day to fight the sire bond.
In Dangerous Liaisons, he calls Caroline, telling her he will come back when he's fixed. He's also mentioned in a conversation between Klaus and Caroline, where she was telling Klaus to give Tyler his freedom back.
In Heart of Darkness, Tyler returns to Mystic Falls after having turned more or less one hundred times in the Appalachian mountains, thus breaking his sire bond. However, he leaves Caroline's house after finding a picture that Klaus gave to her.
In Do Not Go Gentle, Tyler shows up at the dance to "sweep Caroline off her feet". When Klaus arrives and asks to dance with Caroline, Tyler pretends to still be under the influence of the sire bond. This is to prevent Klaus from having any suspicions. Until Bonnie breaks it, Caroline, Tyler and Klaus are unable to leave Mystic Falls High due to the containment spell placed there by Esther. Tyler and Caroline then pay their respects to the dying Alaric.
In From a Cradle to a Grave, after Monique kills Hayley, the latter later transitions into an incomplete Hybrid. The Sire Bond, fueled by motherly instincts, enabled Hayley to instinctively sense the location of her daughter, allowing her to find and re-group with Klaus and Elijah and then later her Sire.
In The Story of My Life, Lizzie awakens and completes her transition to a vampire-witch hybrid by drinking Rupert Vardemus' blood. She seeks out Hope, as her desire to kill Hope is also heightened, but can't physically bring herself to harm her with with the red oak stake. Vardemus suggests trying a non-lethal method, though Lizzie still can't harm her. When Vardemus tried to attack Hope to test a theory, Lizzie stopped him. When Hope realized that Lizzie is sired to her, she made her perform several tasks for the fun of it. She later made Lizzie help her, under the promise that when they are finished, she will send Lizzie on her way and she'll be partially free.
In Not All Those Who Wander Are Lost, Lizzie broke her sire bond to Hope out of sheer will, as she didn't want to kill Aurora. She believed at a fundamental level that it was wrong. She let out a scream when she staked Aurora in the shoulder, purposefully missing her heart. She proceeded to snap Hope's neck and escaped with Aurora. She surmised the reason she could break the sire bond was either because of her bipolar disorder, or that her fear of being alone was outweighed by her desire to not become a monster.
== Vampires ==
: "A vampire only bonds to her sire when she has feelings for him before she turns. Human feelings. Vampirism only heightens those emotions."
: —Nandi to Damon and Stefan in We'll Always Have Bourbon Street
The link between two vampires is different than the link between two hybrids. Whereas the link between two hybrids is formed out of gratitude, the link between vampires seems to be formed out of love. The bond is at its strongest when the Sire is within the presence of the Sired.
When a human loves a vampire, and is turned by that vampire's blood, they may experience the sire bond, due to the emotions that they have felt being amplified by their transition, and do their best to please their sire. Hence, sire bonds between vampires normally aren't platonic, and usually common between those of opposing sexes. The sire bond does not influence feelings, only actions. In My Brother's Keeper, and We'll Always Have Bourbon Street, it was revealed that Elena, as well as Charlotte were sired to Damon, as it was his blood that turned them. Damon also mentions that it is rare among vampires.
It may be possible to break the sire bond the same way hybrids do; hybrid sire bonds are based on gratitude and is broken when they have nothing to be grateful for when they turn over and over again. As long as hybrids are grateful to their master they will be sired; this could also mean as long as vampires are in love with their masters they will be sired. Vampire sire bonds are based on love and is possibly broken when they have nothing to love anymore, i.e. switching off their humanity. It is unknown whether or not the bond is maintained if the sired vampire "falls out of love" with their master. 
Thus there are only two known ways to break a sire bond between vampires. The first is for the sire to tell the one bonded to them to forget about them and move on with their life, as Damon did with Charlotte. The other is for the sired vampire to turn off their humanity, which will break the sire bond. The sired vampire can do this themselves or be told by their sire to do it. In Stand By Me, Damon has Elena turn off her emotions because she couldn't deal with her brother Jeremy's death. It's revealed in Bring It On that this broke the sire bond between him and Elena.
A third possible way was alluded to, that if a vampire (be it the sire or the one sired) took the Cure, the sire bond between them would be broken (since presumably it would change the vampire into a human). This was never confirmed.
== Known Sires ==
* Niklaus Mikaelson - The first Hybrid. Klaus was conceived when his mother had an affair with Ansel - a Werewolf. When Klaus and his siblings were turned into vampires, and made their first kills - so was Klaus, who unknowingly activated his werewolf curse, revealing his mother's affair. Klaus' father, Mikael, set out and killed his wife's lover and his whole family, starting the feud between Vampires and Werewolves which has been perpetuated across the ages. Mikael had his wife put a curse on Klaus that made his werewolf side dormant. But after 1,000 years, Klaus managed to break it, and sired new werewolf-vampire-hybrids from werewolves who activated their curse, and they were all loyal to him through the sire bond - as he took away their pain from having to turn every full moon.
* Damon Salvatore - A 178 year old vampire. He was turned by the vampire Katerina Petrova in 1864 along with his brother Stefan Salvatore. He had two known sired vampires: Charlotte, and Elena Gilbert. Both of them had feelings for him before they were turned into vampires. As a result, they were sired to him right after they were turned.
* Hope Mikaelson - Daughter of the Original Hybrid. Being the first born hybrid, she is an originator of a bloodline, just like her father, step-grandfather, uncles, and aunt. Her blood, which was discovered to be able turn Werewolves into Hybrids, meant those that transitioned because of it are also bound by the Sire Bond coming from her hybrid nature. Her mother, Hayley, exhibited an innate ability to instinctively seek out her child and Sire, in order to complete her transition to a hybrid. After becoming the tribrid, Hope inadvertently sired Lizzie Saltzman. Unlike Hope's other sires, Lizzie did not have to drink Hope's blood to complete her transition to a vampire-witch hybrid, but was sired to Hope.