Anrosis

With an elated shout, the King saw that the wall of fire was moving down the hall, turning the impassible walls of living vines to ash. The knights around him cheered. He turned to look at the mages, but what he saw made his smile fall.

None of the mages said a word, but their young leader simply grimaced at him. As the King looked at the strain apparent on the faces of all the mages, he saw their skin begin to change. Imperceptibly small at first, it started to change more and more, becoming a sort of opalescent crystal that broke away, falling to the ground with a glass-like clink. It didn't take a healer to know that there wouldn't be much left of the mages in a minute or two.

"You knew the whole time, didn't you." The king stated.

"Aye, m'lord. I did. Job needed doing however, and so... here we are." The young mage coughed, sending more of himself crumbling. "Some of the boys should be fine, in the end. See to it the healers get to them, won't you?"

"Of course, my boy. Of course." With a gesture, the King brought a knight over and sent him off to find the healers. The mage coughed again, with each cough seemingly wracking the body all the more, causing the mage to crumble faster. One leg cracked, sending him toppling over. The mage put an arm up to catch himself, but it turned to powder on contact with the ground.

— The Threemetal Cycle, by Ewdin Rickman

Anrosis, also known as "mage death" or "witch's death", is a physical reaction to the mortal form drawing too much magic through itself. It causes the body to break down, crystallizing into a weak, glass-like material that will quickly collapse.

Chronic Anrosis, while not immediately lethal, is incredibly painful as small spots or even entire sections of the body crystallize, becoming painful reminders of the dangers of magic.