The Church's scholars are unsure why certain places become infested with ghosts and devils. Perhaps the sins of men draw them in, or perhaps there are simply places which are more attuned to chaos. Whatever the truth, certain places throughout the World possess a sinister consciousness. It is said that there are mountains that no sane man has returned from, woods that ensorcell loggers to end their own lives, and lakes which call fisherman to drown themselves in their crystalline depths.
Such places need not be natural. Mansions exist which are still haunted by the foul deeds of their former owners. In lost towns devoid of animal life grow obscenely fecund plants. Ghostly lights flit hither and thither about the sites of fierce battles or terrible massacres. Many have met their deaths plying their trade in the shadow of an accursed castle.
For whatever reason, such places seem to call out to those slavering fiends men label monsters. Such places even seem to breed them. The walls themselves drip blood which congeals in the floor to form savage creatures. The fruit of strange trees in deep woods possess slavering maws.
These places are often rich in treasure. The black acts which brought upon such a curse usually also center around vast amounts of wealth. Brave adventurers often try to free golden crosses from the now corrupt altars of ruined temples, but few live to tell the tale.
Since the emphasis of the game is on exploration, I wanted to stress the unusualness of the various locations my players would be exploring. To that end I decided that the dungeons, and primeval forests, and what have you in my setting are at least a tad sapient. This was largely to give an explanation of why doors slam shut the way they do, or how darkness works as per the LBBs.
The other thing this kind of set up allows me to do is give monsters a more nonsensical ecological framework.