Swear to it, make the Unbreakable Vow.
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.