Darkhearts

Maybe three in a generation have the dark mark above their heart, only two were ever unlucky enough to be born with it. It is a bad omen, its bearers often venture on difficult quests to be rid of it. Some say it can be acquired by coming too close to a placespirit, or refusing to become a voice, or travelling the Area.
Darkhearts hold speechcraft power rivaled only by elementarians and the Four. The minds of the dark mark's bearers quickly shatter. They become paranoid, vain, and cruel, all the while impressing an almost inescapable lure on their swiftly growing mass of followers, which they badly mistreat. Almost all darkhearts had to be brought down violently and usually called a placespirit into existence while dying, which laid further waste to the land.

"We loved our adventurous life. When Astinoi, who had always been gloomy and sensitive, discovered the dark mark on himself, that life shattered like glass. We searched feverishly for help, but when it covered his heart fully, he flung himself down the cliffs into the lake."
— Tirakanfar, traveller

That darkhearts are both dangerous and rare make them a scarcely researched subject; on the bright side, darkhearts can't be made on purpose, unlike spirit possession. The cause for their coming and probable cures are largely unknown, and they can't be anticipated or avoided. Until the mark covers the entire heart, there's hope; it doesn't grow by any known rules. Voices will help, by sending darkhearts-to-be on dangerous quests, which often claim their lives; but if successful, they die unblemished.

"Artifacts are hard enough to understand; if made by darkhearts, I wish there was a bottomless pit we could throw them into. Kanmarra's collar alone destroyed half the standing stones in Suyemor in only a day."
— Hinukaya aht Kanamm, whisperer

Artists' notes
Darkhearts put a firmly black spot on Genius Loci's bright and hopeful map. But I like the poor fellows somehow; and there aren't as many as the article may make it sound, mind you: only three per generation, in the entire world. Except that they will be known throughout it if they fully bloom.

Spirit of place - genius loci

"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

Where a mood becomes defining for a place, be it a festival place, city, or battlefield, a genius loci may come into existence. It is an entity of almost limitless power that borders on conscious intelligence - on which side of it is unknown. They rarely appear physically, but fill the horizon when they do, and almost never speak. If they do, they command.
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."
— Headless Nakani, to Queen Jamora

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.
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.

Artists' notes
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.