Episode 26 of The Trigan Empire
The City of Jewels
aka The City of the Jewels
The Zargot Fighter
Het geheim van de eeuwige jeugd (Dutch title)
The Secret of Eternal Youth (Dutch title, translated)
Written by Mike Butterworth
Illustrated by Don Lawrence
It was reprinted in the 1977 Vulcan Annual.
It is reprinted in Volume 7 of The Trigan Empire—The Collection (the Don Lawrence Collection).
It is reprinted in Volume 3 of The Rise and Fall of the Trigan Empire (Rebellion).
Plot summary (Dutch)
MediaFire via British Comics Compilations