Upon a pale horse... (The Mists of Ravenloft)
Kessler is the alpha of a pack of wolfweres that is based in the lands of Invidia. Originally lead by Matton Blanchard, Kessler became the leader when Evan Moreau killed Matton during the siege of Castle Hunadora.
Kessler briefly traveled to Barovia with a group of refugees from Invidia. While their, he and his companion Kira Javed, became enthralled by Count Strahd Von Zarovich. They were eventually freed during an ill-fated attempt to rescue Eli from Castle Ravenloft. They helped defeat the Strahd during that attempt.
In the year following Kessler and Kira have been waging a guerrilla war against the forces of Malacchio Aderre in Invida.