In "The Crimes of Grindelwald", a blood oath prevents Dumbledore and Grindelwald from dueling. However, we know they must have their famous duel in 1945. How can it still happen?
Dumbledore will attempt to destroy it - it’s likely he’ll succeed.
When Newt brings the blood oath vial back to Dumbledore after his Niffler stole it, he asks Dumbledore if it can be destroyed, and Dumbledore says maybe it can.
The vial—blood troth—hangs in the air between them.
Can you destroy it?
Maybe . . . maybe.
- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)
It’s very likely that they’ll succeed in destroying it so Dumbledore can duel Grindelwald in 1945.