Episode 38 of The Trigan Empire
The Secret of Castle Doum
aka The Secret of the Locked Room
Het geheim van de Abandon Burcht (Dutch title)
The Secret of Castle Abandon (Dutch title, translated)
The Secret of Castle Doum was originally published in 9 installments in issues 588 (21 April 1973) to 596 (16 June 1973) of Look and Learn magazine.
It is reprinted in the Don Lawrence Collection Volume 10, The House of the Five Moons.
Plot summary (Dutch)