Shadows of Esteren: The Gaes of Tulg
Dame Gwyn of Donn
Quiet Champion of Smiorail's Fortress
The Demon in the form of Dame Gwyn exhibited the powers of shapeshifting and causing material to rot before its time, and is supposed by the characters to have mental effecting powers to allow it to cause illusions and some sort of precognition.
The characters fought a demon that had taken the form of Dame Gwyn, who claimed that Gwyn was killed more than a year ago.