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 protect Draco, to watch over her son, as he attempted to fulfill the Dark Lord's task, and to fulfill the task if something should happen to him. Another example in Half-Blood Prince occurs when Ron tells Harry how Fred and George tried to make him undertake an Unbreakable Vow, but they did not succeed.

"She flung away her glass; it skidded across the table as she slid off the sofa into a kneeling position at Snape's feet, seized his hand in both of hers, and pressed her lips to it. "If you are there to protect him . . . Severus, will you swear it? Will you make the Unbreakable Vow?" "The Unbreakable Vow?" Snape's expression was blank, unreadable. Snape did not look at Bellatrix. His black eyes were fixed upon Narcissa's tearfilled blue ones as she continued to clutch his hand. "Certainly, Narcissa, I shall make the Unbreakable Vow," he said quietly. "Perhaps your sister will consent to be our Bonder." Bellatrix's mouth fell open. Snape lowered himself so that he was kneeling opposite Narcissa. Beneath Bellatrix's astonished gaze, they grasped right hands."You will need your wand, Bellatrix," said Snape coldly. She drew it, still looking astonished. She stepped forward so that she stood over them, and placed the tip of her wand on their linked hands. Narcissa spoke."Will you, Severus, watch over my son, Draco, as he attempts to fulfill the Dark Lord's wishes?" "I will," said Snape. A thin tongue of brilliant flame issued from the wand and wound its way around their hands like a red-hot wire. "And will you, to the best of your ability, protect him from harm?" "I will," said Snape. A second tongue of flame shot from the wand and interlinked with the first, making a fine, glowing chain. "And, should it prove necessary... if it seems Draco will fail. . ." whispered Narcissa (Snape's hand twitched within hers, but he did not draw away), "will you carry out the deed that the Dark Lord has ordered Draco to perform?" There was a moment's silence. Bellatrix watched, her wand upon their clasped hands, her eyes wide. "I will," said Snape. Bellatrix's astounded face glowed red in the blaze of a third unique of flame, which shot from the wand, twisted with the others, and bound itself thickly around their clasped hands, like a rope, like a fiery snake. "