Cure worse than disease

In $town you may find $father who will tell you about his ill daughter:

My daughter is seriously ill. Heavy chain disease, they say.
She is bed-ridden, barely able to get out of bed and move,
as if bound by a heavy chain indeed.

I've heard that $healer is researching that disease and made some progress.
The treatment is risky but I'm willing to do anything to end my girl's suffering.

She's there in her bedroom, but I'd ask you not to disturb her,
as she's been growing even weaker lately and needs to rest.

Could you talk to that healer and bring me the potion?
I can't go myself since my wife and I need to tend to the ill daughter.
I'll give you a letter about my daughter's condition.

$healer lives in a small house just behind the university campus in the capital.

Making the potion

Talk to the healer, and during this quest her greeting will be “Is it a letter about an ill girl?”. Talk to her about the “ill girl” topic and she will tell you the following:

This sounds like a classic case indeed... I've been working with heavy chain disease victims to find a cure
and so far my most important finding is that it's likely not a single disease,
but rather multiple diseases with similar signs that we don't know how to tell apart.

I also tried to find a cure by taking potion recipes from various herbals and excluding
ingredients that improve one's fatigue and just mask the illness.
I found one combination that actually helps some patients, although it contains
some poisonous ingredients and can be very dangerous to take,
so I only gave it to the most terribly ill pepople who had nothing to lose:
some made at least a temporary recovery, but many still died, and the potion likely
hastened their deaths, even if not caused them.

I'm out of that potion now, but I can tell you the ingredients to find.

All those ingredients are quite obscure and not really easy to find. When you find them, come back to the healer and she will make a potion.

When you bring the potion to the father, he will thank you and reward you with $amount gold. However, the potion will prove fatal for her. Next time you visit the father, he will tell you how his daughter died after taking it.

Discovering the truth

The father says you shouldn’t talk to his daughter, so why not do the opposite? What if he’s hiding something.

If you talk to the daughter, she will confirm that she’s ready for anything to get cured and also that she’s very tired of taking bitter potions to cope with her fatigue.

That will give you a journal entry that mentions bitter potions. The healer will be puzzled by that because she doesn’t know any fatigue-restoring ingredients that would make a potion bitter.

She will suggest to ask the father for a sample and suggest that you ask the daughter to leave a few drops in a bottle.

Go talk to the daughter again, take an empty potion bottle from her and bring it to the healer. The healer will recognize the taste of $plant — a known poisonous plant that causes weakness in small doses. That means the father is slowly poisoning his daughter and likely seeks that heavy chain disease potion because it’s dangerous and can kill her.

Tell any guard in $town about that. The father will be arrested. Talk to the mother who will tell you that the daughter felt much better as soon as she stopped taking poisoned potions and reward you with 2*$amount gold for saving her life.