Helioschosis

Helioschosis is a bacteria that formed on the moon Nimbus that turns its native life to stone when in contact with sunlight. Originally an adaptation to counter solar flares, different forms of life adapted the bacteria into their physiology in different ways. For example, Gargoyles simply benefited from the bacteria, its stone permanently toughening their hides while other species, like the Stone Shark or boulder tortoise adapted behaviors to mitigate or exploit contact with sunlight to avoid petrification.
The Moku people suffered from a debilitating case of helioschosis when the ancient King Laho used his smithing abilities in a failed attempt to graft it to their skin. His actions infected the entire race and regressed them into a stone age society. Most Moku have incorporated the spread of helioschosis into myths regarding their sun Goddess, believing it to be a punishment for their hubris in using technology.
During the Homyn occupation of Nimbus, Count Alyn Arkades developed a cure for Helioschosis with the help of the party. While the Moku rebelled and declared independence from the Homyn Kingdom, their cure is still distributed to those Moku who wish to have it.