Hunter's Curse

“	The hunters were spelled by witches to kill vampires; if you prevent one from fulfilling his destiny then he’ll take you down with him.	”
	— Klaus to Stefan

Elena beginning to experience hallucinations brought by the Curse

The Hunter's Curse is the ability of supernatural hunters that automatically enacts a curse to a Vampire or a hybrid should they kill a member of the Brotherhood of the Five.

This curse causes its victims to be psychologically tortured by the specific hunter even after death by creating powerful hallucinations that torments even the mind of the Original Hybrid, slowly bringing them to a madness that coaxes them into taking their own life.

Workings of the Curse

According to Klaus, the Brotherhood of the Five were originally created to rid the world of immortals, the vampires. Shane added that the death of Silas was their topmost priority. The Hunter's Curse is basically a fail-safe, designed to torment the killer of a hunter and drive them insane so that they could commit suicide, and as a way to protect the hunter from being killed by vampires so they can complete their supernatural destiny. Thus, the hunter's job is still completed. Presumably, the curse itself was meant for Silas, in the event that a Hunter from the Brotherhood were to fall by his hand. 

The Effects of Hunter's Curse

Elena being tormented by Katherine.

The vampire responsible for the murder of the hunter would experience horrifically vivid hallucinations, usually in the form of the people that the recipient has killed in the past (the first being the hunter that was killed to trigger the curse) which would then prey upon the killer's guilty conscience, and may even manifest in strange forms, tempting the killer to kill themselves in order to escape this psychological torment, as was the case of Elena. And even those who are remorseless and have no problem with murder are not exempted, as in the case of Klaus, who was only able to survive because he was an Original, long enough for the curse to eventually stop. The only way for it to end was if a Potential Hunter were to be awakened before the recipient(s) kill themselves.

Vaughn speculates that the curse's existence is not tied with the Hunter's Mark, since he was confident that Damon and Rebekah will bring the Hunter's Curse upon themselves if they kill him, despite the fact that the mark is no longer present on his body. As Silas received the curse after Jeremy's Mark freed him, this further backs up Vaughn's theory. He also seems to think that with the mark having served its purpose and now gone as a result of Bonnie casting the spell that liberated Silas, it will not re-appear again on a Potential Hunter. This means that by a current Hunter of the Five's death, the one(s) responsible for killing him will forever be haunted by the curse, as no potential would be awakened to carry on the predecessor's purpose.

Apparently, the curse is not invoked if the Hunter in question happens to be a ghost, as they are already dead, since Stefan and Alaric both killed Vaughn's ghost without sustaining the curse.

Recipient's of the Hunter's Curse

• Klaus - Klaus got himself unknowingly Cursed after he had slain all the original members of the five in retaliation for their attempt to neutralize his family with White Oak Ash Daggers. He was struck with the hallucinations not long after that, and was tormented inside his head for a period of 52 years, 4 months, and 9 days. It presumably took this long because being the original Brotherhood's killer, the potentials for each member slain had to be awakened at around that time. According to him, it was during this time when he actually experienced the flow of time itself since becoming a vampire.

• Elena Gilbert - Elena sustained the Hunter's Curse after killing Connor Jordan. Klaus, being the only other known recipient of the Curse opted to confine her in order to prevent her from killing herself. She was eventually cured after her brother, Jeremy Gilbert, was deduced by Damon to be the next Hunter of the Five to carry on Connor's legacy, awakened after killing a Hybrid.

• Silas - He got the Hunter's Curse after killing Jeremy Gilbert. However, unlike in the case of Klaus and Elena, he easily defeated it in minutes (as he tells Bonnie Bennett). However, given his deceitful nature, this is debatable.

Trivia

• Shane specifically mentions in We All Go a Little Mad Sometimes, that if a hunter is killed by that which he hunts, then that person will be cursed to walk the earth in torment until a new hunter is awakened and their legacy is passed on. The Curse only affects vampires, the Five's targets. However, hybrids can also be cursed, as claimed by Klaus.
• Silas is the only one who was able to beat the Curse. Exactly why is unknown, but it may be because he is much older than the Brotherhood. Thus, his advanced age and mastery of his psychic powers are too much for the Curse to affect him over a longer period of time.
• Enzo did not sustain the curse after he strangled Jeremy to Death. Though whether or not this was because the Hunter's Mark was no longer present in Jeremy, or the fact that Katherine (in Elena's body) was able to revive Jeremy in time.