Unbreakable Vow

An Unbreakable Vow is a magical spell of unknown incantation, in which one witch or wizard makes an oath to another. If either of the two break their terms, they die.

Casting and effects
"A thin tongue of brilliant flame issued from the wand and wound its way around their hands like a red-hot wire."

- The light of the spell after the first clause.

The Vow is cast by having two parties kneeling opposite each other and clasping right hands. A third must hold their wand, standing quite close to the pair holding hands, and place the tip of their wand onto the linked hands, as Bonder. Then, the first will ask a certain number of vows of the other, with the second accepting those terms; it is unknown what happens if he or she declines. Each time a term is accepted a thin stream of fire will be emitted from the Bonder's wand, weaving around the hands of the pair taking the vows.

1980s
"Fred and George tried to get me to make one when I was about five. I nearly did too, I was holding hands with Fred and everything when Dad found us. He went mental, only time I've ever seen Dad as angry as Mum. Fred reckons his left buttock has never been the same since."

- Ron describing his brothers' attempt to make him use the spell.

In either 1985 or 1986, Fred and George Weasley attempted to convince their then five-year-old brother Ron to make an Unbreakable Vow with them. They were in fact poised to do so, with their wands out, holding hands and ready to cast the spell when their father discovered them. It was described as the only time he was as angry as his wife, and it is said that he did something which left Fred's left buttock never feeling the same way again; however, luckily their brother Ron was left unhurt, although he remembered the incident at least eleven years later.

1996
In the summer of 1996, Severus Snape made an Unbreakable Vow with Narcissa Malfoy, the wife of Lucius Malfoy. Their Bonder was Bellatrix Lestrange, Narcissa's sister and a Death Eater. He promised to protect Draco Malfoy from harm and guide him whilst he attempted to perform the task the Dark Lord had asked him to do: kill Albus Dumbledore.

Behind the scenes

 * It is unknown if this spell has an incantation; although the Bonder has to hold their wand to the participants' hands, it would seem at first as though the vows themselves may have been the incantation. However, it is also possible it was merely cast non-verbally.
 * In the sixth film, the three rings of fire appear all at once, rather than appearing individually whenever a term is accepted. Whether or not more rings would appear if there were more terms to the vow remains unknown. They also leave a mark on the skin once they disappear.
 * Several fans of the series have proposed to their significant others by cutting out pages of the book down to the chapter titled "The Unbreakable Vow", hiding an engagement ring inside the cutout, and giving it to them.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * LEGO Harry Potter: Years 5-7