The People of the pocket realm of Brocschtal are simple folk who live as they have for thousands of years. Farming the land, raising sheep, getting in the occasional brawl. And fighting off the infernal attacks of ghouls.
You find a patch of edible funghi. They taste well and all, no ill effects. UNLESS you consume some beverage, even 3 (three) days after eating it. Then you become really sick with pains, vomiting, all the fancy stuff. As a gift or good meal, it could be a cruel joke or to make sure the heroes spend their time focused on the mission. And the fine soup you had in the inn yesterday could have some in.
(Inspired through a real-world fungus. Was really used to cure alcoholism.)