In the House of Redemption, find the nun, talk to her, and she will ask you to help her obtain 3 Miasma Stones.
Outside the city, it's easy to find 3 ghosts, defeat them and collect their souls to give to the nun.
After completing the above tasks, the nun will tell you the location of the nun leader Elena.
Enter the basement from the Redemption House and go straight to the bottom. Elena may appear, but don't worry about her and continue moving forward.
In the room behind the cloth in the basement, you can find the suspicious potion and case sheet, give them to the old man in the hospital bed. He will tell you to go to the male doctor in the Inn Village.
The doctor is located as shown in the picture. You can also ask the two people on the medical record first. They may be in the slum tavern. Take the ox cart on the left side of the capital to reach the hotel village, find the doctor and give him the medicine.
When you go to the doctor, he will tell you that there is a problem with the medicine and ask you to look for it again.
Return to the Inn Village and find the doctor again. This time he should be able to tell you what the problem is.
Return to the Redemption House again, find Elena, and press the capture button directly. She will be sent to prison, and the mission is completed.