The spread of rumors of the dangers of long-forgotten ruins and places deep in the wilderness, will actually cause those places to become more dangerous for those adventurers who are convinced of that danger. For example, the belief that a ruins is booby-trapped with pit spikes, if believed, will cause the appearance of pit spikes in those ruins; rumors of lack of structural integrity may cause cave-ins the moment an adventurer enters who believes thus. Worse is a rumor that the dangers of the ruins are entirely unpredictable, for that causes dangers to materialize where adventurers least expect it. This process is perilization.
Perilization only works for places that no one has a strong memory of - which is to say, had not been explored anytime recently by anyone still alive. Ruins fall into this category, whereas for example, well-kept castles would not, as the residents of the castle would be well aware of the lack of such rumored perils.
Once the peril has manifested for the adventurers, the adventurers are aware of it; and so they will spread this belief to others, thereby perpetuating the rumor. A rumor that can perpetuate itself in this way is called a meme. And if the ruins claims the lives of some of the adventurers, that only makes the belief even stronger among the rest. And if the ruins claims the lives of all the adventurers, their disappearance will be noticed by others, and their deaths will likely be believed by others to be due to the perils of the ruins, thereby convincing those bystanders of the ruins’ dangers, likewise perpetuating the meme.
The inverse is also true: for those who enter these ruins with a carefree attitude, these ruins will be far less threatening than they would for someone fearful who enters.