In the magical world of Harry Potter series, they do not do things like our way (because we are, mere muggles), they do things the magic way which makes everything all the more interesting and intense.
Blood pact is a very serious form of agreement between two partners. They slit their hands with their wands and let the blood come out. Then they join their hands from the place of cut while addressing their oath. Two drops of blood would rise and merge to form a vial, confirming that the agreement has been made. Blood pact is a very sacred vow which is apparently not destroy able. In the second part of the Fantastic Beasts movie, we see a detailed insight on the relationship of a pair who signs a blood pact.
Deathly hallows are three powerful magical objects, whose possession make a wizard master of death. Dumbledore has two siblings, sister Ariana and brother Aberforth; and studied at Hogwarts. Grindelwald studied at Durmstrang Institute but was expelled later onwards. Dumbledore met Grindelwald while he was on a quest to find the Deathly hallows. Dumbledore is fascinated by Grindelwald plans and Friendships grows between the two of them, Dumbledore becomes an advocate of Grindelwald’s ideology to make wizards authoritative over the muggles.
However, eventually, Dumbledore brother, finds outs his brothers’ activities and the three of them have a quarrel which results in the death of Ariana, which causes Grindelwald to flea the place, and this marks the end of friendship between Dumbledore and Grindelwald.
Dumbledore and Grindelwald are a known case of taking the blood pact. The blood pact they made prevents the two of them from getting into a fight. That is why Dumbledore couldn’t take down Grindelwald, however, due to certain loopholes, apparently there is a way this blood pact can be broken.
Interestingly, the drops of blood are sealed in a necklace pendant that is worn by Grindelwald. Newt Scamander gives Grindelwald necklace pendant to Dumbledore and that is how a hope to destroy the pact arises. m.xcoser.com has a huge collection of Harry Potter artifacts and you can get your hands on this beautiful Grindelwald necklace pendant. It would be a perfect valentine’s gift since it signifies the presence of a sacred bond and trust and any Harry Potter fan would love it.