What is shadowjack?

Some articles on shadowjack:

Jack Of Shadows - Additional Stories of Jack of Shadows
... The Illustrated Roger Zelazny includes a short story, "Shadowjack", that is a prequel to the events of the novel ... Underwood-Miller) reprints the tale "Shadowjack" but without the illustrations ... ed Bill Fawcett, Mayfair Games, 1983 contains the character biography entitled "Shadowjack" ...