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
The City of Jewels was originally published in 9 instalments in issues 480 (3 April 1971) to 488 (22 May 1971) of Look and Learn magazine.
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
Marrus is a famous Zargot fighter, but he’s starting to get old; he’s thinking about retiring. Peric tells Trigo about an old manuscript he has found. It speaks of the City of Jewels. The priests of the city are said to know the secret of eternal youth. The city lies southeast of the Daveli jungle. According to the manuscript, the civilization was destroyed by a volcanic disaster. Trigo decides to mount an expedition to the city. Janno and Marrus also join.
They travel by boat along the river into the Daveli jungle. On the way, the boat capsizes and Marrus saves Trigo’s life. They reach the enormous mountain range where the city is supposed to be located. Marrus takes the first watch, but he disappears. Janno investigates with his plane and discovers the city. He lands there but is captured and ends up in prison with Marrus. They are brought to the hall of leaders, all of whom are young men. They are sentenced to forced labor. However, Trigo frees them, and they join the rebels of the city.
The city’s inhabitants have also been oppressed for years and see Trigo as a liberator. They want to march against the leaders to overthrow them, but Marrus—who wants to learn the secret of eternal youth—betrays them in exchange for the secret. During the uprising, they walk into a trap, and the rebels and Trigo are overpowered. The leaders, however, do not tell Marrus the secret and intend to kill him. Just as they are about to strike him down, Trigo jumps in and saves Marrus. Marrus now becomes furious and charges at the leaders. He discovers they are actually old men wearing masks of young men’s faces. The rebels seize the opportunity, and the leaders are overpowered. The secret of eternal youth was just a myth to keep the people under control. Trigo forgives Marrus and asks him to accept that he is growing older.

Links
Comicweb (Dutch)