Blood pact

A blood pact is the magical equivalent of a blood oath. In the Wizarding world, a blood pact serves as a covenant that constitutes a binding magical contract.They will then use their wands to make a cut across the palm of their hands and, while still bleeding, interlace their hands. Upon doing this, two glowing drops of blood will raise from their palms, which will come together and mingle to grow one. Thereafter, a vial will take shape around it, enclosing the drop, signifying that the deal was struck. Once made, the involved parties are apparently physically unable to do anything that went against the conditions set for said agreement.

Once made, it was apparently difficult to destroy, as even Albus Dumbledore expressed uncertainty as to whether or not he would be able to do so when confronted with the question by Newt Scamander.

History
During their teenage years, Albus Dumbledore and Gellert Grindelwald made a blood pact in a barn, presumably somewhere in Godric's Hollow, where they swore an oath never to fight each other.

Grindelwald would carry the vial on his person at all time, using it as leverage to prevent Dumbledore from going after him during the Global wizarding war. A two-egged sword, however, it also prevented him from removing the biggest threat to his cause from the playing field, prompting him to seek out a powerful Obscurial as to have a weapon in the form of a henchman powerful enough to kill Dumbledore for him. 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.

Scamander brought it to Dumbledore at Hogwarts shortly thereafter, so he could destroy it.