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.)
The party has found the source of the strange creatures roaming the countryside. The rift in this reality glows with a silver hue, rippling with the wind but never moving. They step through and are immediately assaulted with the scent of rotting meat, some have to muster all their strength not to vomit. Strange cries similar to the beasts the party had faced before can be heard in the distance. Looking around, they see they are in a forest of grey and red rather than the normal brown and green. The trees are sticky to the touch and writhe, perhaps to get away or perhaps as a warning.
The deeper the party goes, the more the forest seems to slither and move underfoot. The cries get closer and more numerous. Creatures lurk in the shadows, all the same color of their surroundings. Whatever the party came in here for, they had better do it fast.
The forest of flesh is waking up, and it is so very hungry.