Tiaral

But I don't believe that the Foglings cultivated some super-poisonous fruit for which only they can create an antidote, while the second most influential clan cannot.

What's so unbelievable about that? They either took or selectively bred an initially poisonous fruit and treated it with pesticides. For good measure, they added a secret cheese ingredient from a cook, which is difficult to identify blindly in a laboratory within a timeframe sufficient for saving someone. If we abstract away from this damned medieval setting and imagine that the antidote here is something more complex than some kind of dietary supplement or ground-up plantain in grandma's mortar.