- Harry: "...what does it mean?"
- Ron: "Well, you can't break an Unbreakable Vow..."
- Harry: "I'd worked that much out for myself, funnily enough."
- — Harry Potter and Ron Weasley in 1996[src]
An Unbreakable Vow is a magical incantation in which one wizard makes an oath to another. If the person who accepts the conditions of the Unbreakable Vow breaks them, they die. The spell involves the two wizards joining their hands, and a third person, designated as a "Bonder", placing the tip of their wand onto their hands, upon which one person asks another person three separated terms of the Vow, where the person will respond "I will". Upon assent with each clause of an Unbreakable Vow, a thick tongue of fire winds around the linked hands.
Known Uses
Severus Snape and Narcissa Malfoy make an Unbreakable Vow with Bellatrix Lestrange as their Bonder.
1980s
In either 1985 or 1986, Fred and George Weasley tried to get their five-year-old brother Ron to make an Unbreakable Vow. They were poised to do so, with their wands out and about to cast the spell, but their father found out what they were doing and stopped them. Ron described it as the only time Arthur Weasley was as angry as his wife, and said that he did something that left Fred's left buttock never feeling the same way again.
1990s
In 1996, Severus Snape made an Unbreakable Vow to Narcissa Malfoy, with Bellatrix Lestrange, Narcissa's sister, serving as their Bonder, with Snape vowing to watch over and protect Narcissa's son Draco from harm as he attempts to fulfill the Dark Lord's wishes, and, if necessary, carry out the deed the Dark Lord ordered Draco to perform: kill Albus Dumbledore. Snape fulfilled his vow by killing Dumbledore during the Battle of the Astronomy Tower (secretly at the fatally-injured Dumbledore's request, in the course of Snape's duties as a double-agent in the Death Eaters ranks).