This has been driving me nuts. It was a color movie in English probably from the 1980s. At least, I think it was a movie because in my memory it seemed too long for a TV show episode.
Three people in the woods (maybe four – there is at least the main “hero”, a woman and a heavier-set man) and they find a large building (I think made of stone) and go inside. They see something inside that makes them realize if they kill any animals before they leave they won’t be able to get out again. (The hero may have a sword?) Should be easy, but as they are walking out the female slaps her neck and kills a bug. I remember very little other than that scene, but do know they get stuck in the building, separated and all kinds of weird things happen while they are in there.
Because of the animal thing I always used to think maybe it was related to Beastmaster, but I’ve checked those and it doesn’t seem to match up.