Sinding

 is a werewolf imprisoned in the jail of Falkreath.

Interactions
Upon speaking to Mathies, he explains that Sinding killed a little girl while in werewolf form. The girl was the daughter of Mathies and Indara Caerellia, who both live at Corpselight Farm in Falkreath.

Sinding was in Falkreath because he was looking for a giant beast roaming the wilds around the village. If he killed this beast, the Daedric Lord Hircine would lift his curse.

During the quest "Ill Met By Moonlight", the Dragonborn receives the Ring of Hircine from Sinding, which causes the wearer to sporadically transform into a werewolf. The transformations only occur if the Dragonborn is already afflicted by Lycanthropy, and the ring cannot be removed until the quest is completed. When walking away from his cell a glance backwards will show a werewolf ripping off the bars from the roof, and guards will ask you about him escaping when leaving the barracks.

If the Dragonborn kills Sinding and takes his skin, Hircine appears again to award him or her with the Savior's Hide and also removes the Cursed Ring Of Hircine from the inventory. If the Dragonborn kills the hunters, however, Hircine appears and un-curses his ring, freely allowing to transform into a werewolf at will. A third choice allows the Dragonborn to get both the Ring of Hircine as well as the Saviour's Hide by siding with Sinding first, then killing and skinning him after Hircine removes the curse from the ring, to receive the Hide as well. This means you must exit the area, talk to Hircine just outside the entrance and return to where you left werewolf Sinding, thus circling Bloated Man's Grotto twice. This can be performed much later in the game as Sinding can be found running a path from his prison cell to Bloated Grotto long after you perform the quest and spare him.

It seems Sinding is ultimately unable to control his beast blood. If the option to spare Sinding is taken, even if the player turned around and killed him as well to complete the quest both ways, he can be encountered later as a random encounter in a hostile werewolf state fighting a guard from the local hold. He can also (having spared him) be encountered in werewolf form, but friendly, in Bloated Man's Grotto.

Quests
 * Ill Met By Moonlight