What if Ariana Dumbledore never died

Why is Graves so desperate to find an obscurial?

Graves / Grindelwald wanted to find an obscurial really bad because Grindelwald believes that only a really powerful obscurial can kill Dumbledore.

I say powerful because obscurials are hardly older than 10 years. But Credence is reaching his 20s, which makes him an extremely powerful obscurial, capable of extreme destruction.

In "The Crimes of Grindelwald" it is revealed that Grindelwald believes that Credence is the one who can defeat Dumbledore.

Claw Why is it so important?

GRINDELWALD goes to KRALL.

GRINDELWALD Who is the greatest threat to our cause?

Claw Albus Dumbledore.

GRINDELWALD Now if I asked you to go to the school he is hiding in and kill him for me, would you do it for me, Krall? (smiles) Credence is the only living thing ... that can kill him.

KRALL you believe you really think he can kill the great - can he kill Albus Dumbledore?

GRINDELWALD(whispers) I know he can. But will you be with us when that happens, Krall? Will you?

Fantastic Beasts: The Crimes of Grindelwald - The Official Screenplay , Scene 46

When Grindelwald learns of "a powerful obscurial in New York", he goes there to find the obscurial. Without knowing that Credence itself is the Obscurial, Grindelwald asks Credence for help in finding the 'Obscurial'. As soon as he learns that Credence is the Obscurial, he tries to gain his trust, which ends badly for Grindelwald in Part 1 (everyone thinks Credence is dead by the end of Part 1). Grindelwald then tries to get Credence into his ranks in Paris so he can ambush Dumbledore with Credence.

Additional fact:

Why can't Grindelwald go against Dumbledore himself and need Credence / Obscurial? Because Grindelwald and Dumbledore made a blood pact so as not to fight each other. So they can't fight each other even if they wanted to. That's why Grindelwald wanted to achieve this with an extremely powerful obscurial.

TheLethalCarrot

However, it's worth noting that it's not clear whether Grindelwald believes Credence can defeat Dumbeldore because he is a powerful Obscurial or because he really is.

Anu7

I firmly believe in it because Grindelwald believes that an 'entity' can defeat Dumbledore - what Grindelwald sees him - not for who he is, but for what he is. That said, Credence is extremely powerful because he is an obscurialist who lived through his adulthood. I believe that Credence as Dumbledore is a piece of a very manipulative Grindelwald.