Por que Graves quer encontrar um Obscurial tão mal?

Graves / Grindelwald queria encontrar um Obscurial muito mal porque Grindelwald acredita que apenas um Obscurial realmente poderoso pode matar Dumbledore.

Eu digo poderoso porque Obscuriais mal passam dos 10 anos de idade. Mas Credence atinge seus 20 anos, o que faz dele um extremamente poderoso Obscurial, capaz de destruição extrema.

Em 'The Crimes of Grindelwald', é revelado que Grindelwald acredita que Credence é quem pode derrotar Dumbledore.

KRALL Why is he so important?

GRINDELWALD walks to face KRALL.

GRINDELWALD Who represents the greatest threat to our cause?

KRALL Albus Dumbledore.

GRINDELWALD If I asked you now to go to the school where he is hiding and kill him for me, would you do it for me, Krall? (smiles) Credence is the only entity alive... who can kill him.

KRALL You really think that he can kill the great—can 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}

Então, quando Grindelwald descobre "um poderoso Obscurial em Nova York", ele vai até lá para encontrar o Obscurial. Sem saber que o próprio Credence é o Obscurial, Grindelwald pede a ajuda de Credence para encontrar o "Obscurial". Uma vez que ele saiba que Credence é o Obscurial, ele tenta ganhar sua confiança que acaba mal para Grindelwald na Parte 1 (todo mundo acha que Credence está morto no final da parte 1). Grindelwald então tenta colocar Credence em suas fileiras em Paris, para que ele possa desertar de Dumbledore usando o Credence.

Fato adicional:

Provavelmente por causa de Ariana Dumbledore.

Em Harry Potter e as Relíquias da Morte , nós aprendemos sobre Ariana Dumbledore, a irmã de Albus.
Enquanto o termo "Obscurus" ou "Obscurial" nunca é mencionado na série original, a descrição de um Obscurial coincide com a descrição dada por Ariana.

JACOB: Can someone please tell me what this Obscurial - Obscurus - thing is? Please?

TINA: There hasn't been one in centuries.

NEWT: I met one in Sudan three months ago. There used to be more of them, but they still exist. Before wizards went underground, when we were still being hunted by Muggles, young wizards and witches sometimes tried to suppress their magic to avoid persecution. Instead of learning to harness or to control their powers, they developed what was called an Obscurus. It's an unstable, uncontrollable Dark force that bursts out an attacks... and then vanishes.
_{Fantastic Beasts and Where to Find Them (2016) (source)}

Aberforth glared at her: His lips moved as if he was chewing the words he was holding back. Then he burst into speech.
"When my sister was six years old, she was attacked, set upon, by three Muggle boys. They'd seen her doing magic, spying through the back garden hedge: She was a kid, she couldn't control it, no witch or wizard can at that age. What they saw scared them, I expect. They forced their way through the hedge, and when she couldn't show them the trick, they got a little carried away trying to stop the little freak doing it."
[...]
"It destroyed her, what they did: She was never right again. She wouldn't use magic, but she couldn't get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn't control it, and at times she was strange and dangerous. But mostly she was sweet and scared and harmless.["]
_{*Harry Potter and the Deathly Hallows, chapter 28: "The Missing Mirror"}

Gellert estava familiarizado com Ariana e estava lá quando ela morreu. Isso provavelmente lhe dava uma idéia de quão poderoso e obscuro era Obcuro, e talvez também lhe desse o desejo de evitar mais desses casos; afinal de contas, ele era inteiramente pró-bruxo, e ter um bruxo morrendo cedo por causa da repressão da magia - sendo causado por trouxas - teria sido algo contra o qual Grindelwald seria totalmente contra. E assim vemos que Graves - também conhecido como Grindelwald disfarçado - queria salvar os Obscurials - como Chastity, a irmã de Credence, que ele originalmente achava ser o Obscurial, e o próprio Credence, uma vez que ele foi revelado ser o Obscuro.