Dark Sun: The Tyrant of Tyr
he is quite handsome with long black hair, but now he is extremely thin and malnourished and his hair is flat and oily, the rags he wears barely conceal his bony frame and he visibly shakes
Yrkoon has been helping the sky singers to increase growth rates of creatures and insects, so that they would be ready to bear young, mate and mature as infants far faster than any other house, using a ritual he found in an ancient ruin as part of a raid.
He was present when the attacks on the sky singers took place in Nibernay. They came out of nowhere, blending into the shadows as if they were not there, then they struck and before he knew it everyone had gone, except him. It was then that he was bound and blindfolded and taken to Fort Iron
He was forced to work for Clavius and his cronies and show them how the Sky Singers were able to increase the growth rates of animals.
He was able to trick them however, with disguised spells and illusions to make it look like the animals had grown, when in fact they were the same. It had worked for a while, but he could tell Clavius was becoming suspicious and one day he snapped, tossing him into a pit in the bowels of the tower and leaving him to rot for a while before trying again. It is down the pit that the PCs found him, shaken and malnourished.