"The city of Hammerfall only exists because of a spleen by the mountain spirits in the first place. More and more signs have arisen that it may, and soon, become the host of a placespirit - an event that, in regard of the city's size, must be averted at all costs."
— Counselor Tshiako Sagamiu
The placespirit will possess anyone in its domain. The victims may dance until they drop dead, start erecting uninhabitable palaces, or dig a shaft. As unreasonable as all placespirits' commands seem, time and again the results have been part of turning points in history.
"It is the Violet Order's belief that the placespirits must be thoroughly researched, seeing as the survivors carry an uncanny wisdom out of their service to them."An unchallenged genius loci can stay indefinitely, and must be driven out. This difficult task can only be performed by highly skilled speakers or collectors; there are very, very few who ever drove out a placespirit alone, among them the famous voice Asa and the collector Numkrichtahn.
— Headless Nakani, to Queen Jamora
The saved victims will, in any case, not be the same and may act irrationally on a whim years into the future, but to allow them the appearance of normality, they are often brought elsewhere to pick up their lives.
The name-giving spirit of place (Latin: genius loci, German: Ortsgeist) is an important part of the setting, of course. It's more or less my remedy for not having dragons or orcs handy to devastate a land - instead the population will be mentally enslaved by a terrible willpower... uh-oh.
Since they are so hard to research, many strange folk have obsessed over them, and suffered horrible fates - different every time, there just seems to be no rule to placespirits.