Episode 5 of The Trigan Empire
The Land of No Return
aka The Legend of Hellas
Het rijk der laatste dagen (Dutch title)
The Land of the Last Days (Dutch title, translated)
Written by Mike Butterworth
Illustrated by Don Lawrence
The Land of No Return was originally published in 5 instalments in issues 238 (6 August 1966) to 242 (3 September 1966) of Look and Learn magazine.
It was reprinted in 1975 in Vulcan comic.
It was reprinted in 1978 in The Trigan Empire (Hamlyn/Chartwell).
It is reprinted in Volume 1 of The Trigan Empire—The Collection (the Don Lawrence Collection).
It is reprinted in Volume 1 of The Rise and Fall of the Trigan Empire (Rebellion).

Plot summary
Peric makes his final journey across the Pyx in a Hellas boat. This river ends in a large cave into which Peric disappears.
One year later, Janno and Keren are practicing their aerial combat. However, Janno crashes into the Pyx River and disappears into the cave. There, he is captured by two soldiers and enslaved. He meets Peric, who is also a slave.
In the enormous cave, a beautiful city has been built. This is where the Zerds live. For a thousand years, they have enslaved those who complete their final journey.
After some time, Janno takes control of an anti-gravity device belonging to the Zerds. He detonates it on the other side of the cave, where the Pyx continues its flow through a tunnel leading to an underground lake. Because this tunnel is now blocked, the cave fills with water, allowing Janno, Peric and Keren to escape.
Links
https://www.2dgalleries.com/art/the-trigan-empire-the-land-of-no-return-page-127-72909
Plot summary (Dutch)
Downloads
British Comics Compilations [PDF]