Magic force field ? Or some gaz or magic that disable, give pain or kill when aproaching or entering it, without anybody knowing particularly that "a gaz" is the problem; just that whoever enters get sicks as hell and cannot move anymore
(If want to have a reason why nobody never entered, but your players needs to be able to enter)
Please put emojis or pictures in your posts & comments ! (But please also avoid 4k sized pictues, most of lemmy instances have bad CSS (displaying nicely things) for the moment)