In order to find more information about Ciri, you have been sent to Downwarren to perform a task for the Crones of Crookback Bog.
After speaking to the ealdorman head to the southeast towards the Whispering Hillock. Proceed to the gnarled-looking tree and activate your Witcher Senses.
Head northwest up the incline and defeat the werewolf. If the werewolf is not taken out quickly, it will regenerate its health when it glows red. Look to the left to find a rock covered in paint, climb up and enter the cave. There is also a place of power adjacent to the cave entrance.
There will be two paths to take; the left is blocked and can’t be passed yet. Take the right path and dive into the water, following the passage until you emerge at the end. Once on dry land, you will eventually find a tree with a beating heart. You have two choices at this point, doubt it and fight Endrega to resume the quest or agree to help, which leads to a hunt for ingredients.
If you chose to fight it, you will have to battle waves of Endrega while intermittently attacking the heart. As soon as a wave ends you should begin attacking the branches in front of the heart until they disappear, and then the heart itself.
If you decide to help, you will need to gather a ravens feather, a black horse, and some bones. Exit the cave and follow the quest marker west to a small swamp where you fight a water hag. Search the southern part of the area indicated by your minimap with your Witcher Senses to locate a tiny grave and loot it. You should already have the ravens feather from the harpies nest in Ladies of the Wood. Now follow the quest marker again until you locate a group of wild horses. You will need to approach one slowly and use the Axii spell to gain control and mount it. Go back to the heart of the tree, riding the horse and carrying the contents of the grave.
After reaching the spirit, a cutscene will occur where you can make your final choice, freeing the spirit, tricking the spirit into a conflict-free death, or engaging it in battle. If you wish to free the spirit, simply complete the ritual as directed. In case you want to trick it, go through the ritual but do not stab the heart. This will kill the spirit without Geralt having to battle any Endrega. The final option of battling it has the same result as choosing to fight it before gathering the ingredients.
Following your confrontation with the spirit in the tree, you will be able to finish up Ladies of the Wood.