Blood pact



A blood pact was a covenant between wizards that constituted a binding, magical contract. It could be made by individuals cutting their hands with their wands and interlacing them. Upon doing this, two glowing drops of blood would raise from their palms and mingle into one. Thereafter, a vial would take shape around it, enclosing the drop, signifying that the deal was struck. Once made, the vial was perhaps impossible to destroy, though Albus Dumbledore was willing to try to destroy the one he had once made with Gellert Grindelwald.

Dumbledore and Grindelwald
During their teenage years, Albus Dumbledore and Gellert Grindelwald made a blood pact in a barn where they swore an oath never to fight each other. By 1927, Grindelwald possessed the vial for this pact, which he used to prevent Dumbledore from trying to destroy it so he can fight him directly. However, the pact was a double-edged sword as while it protected Grindelwald from Dumbledore, it also prevented him from eliminating the greatest threat to his cause as he too could not attempt to personally kill his former friend. Although it served a purpose for it left Dumbledore unable to fight Grindelwald, therefore making Grindelwald unstoppable personally as the only wizard capable of rivaling and surpassing his might at the time couldn't directly challenge him, Grindelwald was not satisfied as he knew fully well that Dumbledore could just defeat any of his allies and still intervene in his plans anyway and not underestimating what his minions considered a minor setback, he tried to find a way to overcome it. Eventually, Grindelwald discovered that the extremely powerful Obscurus Aurelius Dumbledore had the potential to be capable of killing Dumbledore and planned to use him as a weapon against Dumbledore. The vial was stolen from him, however, without his knowledge by a Niffler belonging to Newt Scamander during the Battle in Lestrange Family Mausoleum of 1927, at the Père Lachaise Cemetery in Paris, France, although at the time Grindelwald already had Credence and began molding him against Dumbledore. Scamander brought it to Dumbledore at Hogwarts shortly thereafter, so he could destroy it. Although it apparently took years, Dumbledore clearly managed to abolish the blood pact, allowing him to finally defeat Grindelwald in a legendary duel.

Behind the scenes

 * The nature of the restrictions of the pact or possible consequences for breaking it are unknown.
 * The timing of Grindelwald and Dumbledore's pact is unknown and, if made in advance, whether it should have prevented the three-way duel between Grindelwald, Aberforth, and Albus Dumbledore that resulted in the death of Ariana Dumbledore.
 * It is currently unknown, but possible that Ariana's death was a consequence of Albus breaking the blood pact by trying to defend Aberforth from Gellert.
 * Considering how Dumbledore seemed unsure he could destroy the blood pact between him and Grindelwald and Grindelwald having never tried to destroy the vial containing the blood that sealed their oath despite his already extraordinarily powerful magic being enhanced by the Elder Wand, it is currently unknown how Dumbledore was able to fight Grindelwald in their legendary duel. It could be that Dumbledore searched information on blood pacts and after presumably years of research, eventually found a way to terminate it, which could explain why it took him so long to finally engage Grindelwald in one-on-one magical combat.
 * In many cultures, blood is seen as a prevailing symbol of life. The tradition to affirm a contract, alliances or vow in blood, was regarded as making the deal irreversible in nature, and involved a solemn ritual that involved the mixing of two bloods as a sacred seal.