The Unbreakable Vow is a voluntary agreement made between two wizards or witches. It must be performed with a witness ("Bonder") on hand, holding their wand on the agreeing persons' linked hands to bind them with magic in the form of a tongue of flame. The Vow is not literally "unbreakable" as the person taking it is still able to go back on his/her word, but doing so will cause instant death. The Unbreakable Vow was first introduced in Half-Blood Prince, in which Snape made a promise to Narcissa Malfoy to carry out her son's, Draco, task should he fail.