Warning, spoilers for Fantastic Beasts: The Crimes of Grindelwald ahead!
When we jump into Fantastic Beasts: The Crimes of Grindelwald, it's only been a few months since the events of Fantastic Beasts and Where to Find Them. After Percival Graves (Colin Farrell) is revealed by Newt (Eddie Redmayne) and Tina (Katherine Waterston) to be Gellert Grindelwald (Johnny Depp) in disguise, he's taken into custody by the American Ministry of Magic. In The Crimes of Grindelwald, the dark wizard escapes custody and travels to Europe to begin amassing a dark army.
The main crux of the film surrounds the mad dash to prevent Grindelwald from finding Credence (Ezra Miller) and harnessing the dark power of his Obscurus. Newt, who is just trying to live his best life with his magical creatures, is tasked with the job of locating Credence by the one and only Albus Dumbledore (Jude Law). While Dumbledore tries to convince Newt to travel to Paris illegally, the younger wizard raises the very important question of why the Hogwarts professor won't go after Grindelwald himself. Even though he's still pretty young in The Crimes of Grindelwald, Dumbledore is widely considered one of the most powerful wizards around. With his intimate knowledge of how the dark wizard works, one would expect Dumbledore to take it upon himself to stop his former "friend" rather than endanger the life of his former student. Turns out it's a little more complicated than that.
Throughout the film, we're given no true hint to why Dumbledore won't fight Grindelwald himself. Even the Department of Magical Law Enforcement finds his lack of action suspect, going as far as to track his magic to see what spells he uses until they can find the dark wizard. As Newt, Tina, and Jacob race to find Credence before Grindelwald can, Dumbledore takes a trip down memory lane with the Mirror of Erised. As he stares into his reflection, he sees the face of Grindelwald appear. Then the mirror shows a flashback to the two of them as young men, slashing their palms and clasping hands in what we discover is a blood oath. A vial emerges from their joined hands that contains their blood, which the current Grindelwald constantly wears on his person.
As Dumbledore explains to Newt at the end of the film, the blood oath was to swear that they would never fight one another. Considering how neither wizard has made a move against the other, it's fair to assume that a blood oath is similar to an Unbreakable Vow — if either of the two breaks the terms, they die. Newt is able to steal the vial with the help of his friendly Niffler, though Dumbledore admits he doesn't know how to break it. As the film concludes, the two head into Hogwarts for a cup of a tea and some serious brainstorming.
So, even though Dumbledore doesn't attempt to fight Grindelwald in this Fantastic Beasts installment, at least he has solid reasoning. While we don't know exactly how he and Newt are going to solve the issue of the blood oath, we do know they eventually figure it out. As all fans of Harry Potter know, Dumbledore and Grindelwald face off in 1945, engaging in what is stated as "one of the greatest duels ever fought between wizards." Dumbledore becomes the new master of the Elder Wand, and Grindelwald is imprisoned in Nurmengard until he's killed by Voldermort in 1998. Which means the remaining films will likely set the stage for the infamous battle, with its conclusion featuring in the final film of the series.