Episode 31 of The Trigan Empire
The Miniature Killers of Zelph
aka The Black Duke
De zwarte hertog (Dutch title)
The Black Duke (Dutch title, translated)
The Miniature Killers of Zelph was originally published in 8 installments in issues 527 (19 February 1972) to 534 (8 April 1972) of Look and Learn magazine.
It is reprinted in the Don Lawrence Collection Volume 8, The Prisoner of Zerss.
Plot summary (Dutch)