Mr. Jester is an early story from Fred Saberhagen’s berserker series. Appearing in IF magazine in 1966, the story was included in the first collection of Fred’s short work ‘Berserker’. (‘Berserker’ is published in e-format by JSS Literary Productions and is available for Kindle, Nook and iBook.) Mr. Jester also was included in Fred’s 1981 collection ‘Berserker Wars’ and in Fred’s 1998 megabook from Baen Books ‘Berserkers: The Beginning’. Blackstone Audio produced an audio CD edition and an audible downloadable edition of ‘Berserkers: The Beginning’ which, as mentioned, includes Mr. Jester.

Fred’s official website and its mirror:

A great berserker fansite by cognitive scientist Jason Finley