How to unlock the quest
You can obtain this quest by visiting the hut of hermit Pinastri and talking to the lady. The hut is in the eastern part of the kingdom, close to the Dun Tynne Crossroads (M3,9) road sign. Listen to her problems with the invisible phantom that has been haunting her. You can tell her that you will look into it right away, or negotiate the contract terms first. The woman does not have any gold on her, but she offers you a reward in the form of potatoes. You can either accept this form of payment, or help her for free.
Find the phantom
Start by examining the hut's interior. Find a note (The Flagellant's Manifesto), find the hallucinogenic mushrooms, the bloody pillow and the locked window. Leave the hut and examine the dead moths on the ground, by the window. Talk to the hermit again, who gives you Common Graytop Brew. Drink it.
After you regain consciousness, Geralt will find himself experiencing hallucinations and, among other things, able to talk to Roach. Start by exploring the surroundings of the hut on horseback. You do not have to do anything in particular, just wait for Roach to find more tracks. After that, discuss all of the discoveries that you have made so far.
Umbra will soon appear near the hermit's house. Mount Roach and start chasing the monster. The chase in itself is not too demanding and even when Roach needs a moment to catch its breath, you do not need to worry that you will lose the monster.
Defeat the apparitions
The chase ends in a location with a large tomb. Dismount the horse and prepare for Apparitions to appear. Their looks and behavior resemble those of panthers, which is why you should use the same strategy here. You should especially remember about frequent dodging to avoid their claws.
After the battle is over, approach the tomb and examine it, prompting the spirit of Marcell Clerici to appear. You can listen to what he has to say and decide what you want to do about it.
Variant A - Forgive the spirit for its sins
IF you are convinced that Marcello deserves to be forgiven, you have to select the first dialogue option twice. Roach will accept the knight's apology. In this variant, the spirit disappears and you do not have to fight it. You can report back to the hermit. If you have earlier decided to take your reward from her, you will receive potatoes.
Variant B - Reject the spirit's apology
If you are convinced that Marcello deserves no mercy, you have to select the second dialogue option. As a result, you will have to fight Knight's Spirit. During the fight, you can use standard sword attacks optionally supplemented with oil against apparitions and / or e.g. Igni sign, that will set the knight on fire. After the fight, return to the hermit and tell her the whole story. If you have earlier decided to take your reward from her, you will receive potatoes.