Important: this quest has a small time window: it only becomes available after finishing the main quest, La Cage au Fou, and must be completed before finishing the next main quest, Where Children Toil, Toys Waste Away, or else it will no longer be available.
After the quest La Cage au Fou, go to Count de la Croix's Mill to overhear some peasants talking about running off with flour from the mill. As soon as they see you, they'll attack, so take care of them, which will trigger the quest. Head inside the mill to find footprints leading upstairs. Follow them up to discover a large blood stain on the top floor and Geralt will conclude his findings.
- In the vision Geralt had experienced after imbibing Resonance, he had also seen a mill. He soon reached this site and explored it thoroughly. After all, Dettlaff had recently been there himself. The witcher hoped to find clues that would lead him to the murderer.
- If Geralt investigates completely:
- Using his Witcher Senses, Geralt thoroughly searched the mill. It had been the property of one Count de la Croix, who had put it up for sale. And it was here that the unfortunate count had met his end, murdered in cold blood.
- If Geralt doesn't investigate completely:
- The witcher had found some interesting clues, yet he decided to continue the investigation elsewhere. He thus abandoned the lead he had found, related to Dettlaff's visit to the mill.
- Use your Witcher Senses to search the mill for clues.
- The quest can sometimes be bugged, especially if you have been exploring the mill and looting the items inside before starting it. In that case, it won't complete after examining the bloodstain. It will essentially be stuck and ultimately fail once you progress the main story.
- If you want to avoid failing the quest, you can either reload a save from before exploring the mill, or (since you may have played for quite some time between doing so and completing La Cage au Fou) not trigger it in the first place.