The Whispering Hillock - Velen
This is a short quest given to you after discovering the Ancient Oak and before completing the Ladies of the Wood quest. You're given two options for freeing the spirit trapped in the Ancient Oak and each option massively effects the story line for the Bloody Baron, Downwarrens and the Crones.
Before you make your decision, here are the consequences for your actions:
Fighting the Spirit/Defeating It: Downwarren is safe, the orphans in the bog are eaten by the Crones. Anna remains alive but goes insane and is taken by the Bloody Baron to get "help".
Freeing the Spirit: The Orphans are saved and move to live in Novigrad; however the Spirit will attack Downwarren and kill most of the villagers. Anna(Grans) is punished for losing the children and cursed to be a Water Hag. When Geralt removes the curse from Anna she dies and the Bloody Baron commits suicide over the loss.
Alternative (Free Spirit Before Consulting Ealdorman): The village will still be attacked by the Spirit but Anna does not get turned into a Water Hag. The orphans seem to die as well as they aren't in Novigrad later in the game. Last but not least, Return to Crookbag Bog quest has many odd consequences.
If you choose to defeat the Spirit there will be waves of enemies that come out to attack you, during which you'll also have to attack the heart trying to defeat it. Saving the Spirit will require you to collect two or three items (depending on if you've done the Ladies of the Wood quest).
You will need to bring the Spirit these three items:
Raven Feathers: Found in the same Harpy Nest that the bottle was in for Johnny's Voice in Ladies of the Wood. If you've done this quest you probably already have the feather.
Spirit's Bones: Go over to the quest marker on your map and use your Witcher Senses to search for a Gravestone.
Black Horse: Go to the location marked on your map and use Axii on a horse to charm it, then ride it back to the Ancient Oak.
That's all there is to this quest, it's a rather short quest but with major reprocussions to the story line of the Bloody Baron. Probably designed as its own quest on purpose to control all of the variables better. Before leaving the Ancient Oak be sure to search the remains of the Spirit's original body, they'll contain Diamond Dust.