Gath Rimmon is a Dead World that was once the home of the ancient Eldar artefact called the Ayex Commoragh, also known as the Heart of Agony. The planet suffers from acidic floods, which can last for entire years. The only life on the planet are obsidian-skinned xenos that are hostile to any trespassers.
The Ayex Commoragh was hidden on Gath Rimmon by the Adeptus Mechanicus Tech-priest and Logis Alsmo Charis. An Eldar Warlock by the name of Valiel and a squad of Striking Scorpions hunted Charis down to this world, but were destroyed by the Heart. In his last moments, Valiel left a psychic beacon on the roof of the cavern where the Heart was locked away. Years later, a squad of Space Wolves, led by the Rune Priest Ravenblade, were drawn to the world. Ravenblade and Baldr Svelok, a Wolf Guard, managed to destroy the Heart by flooding the chamber with acid at the end of one of the planet's flood cycles. Ravenblade died shortly after as a result of his injuries, leaving the now one-armed Svelok as the only survivor. The rest of the squad had been killed by the Heart shortly after Charis regained control of it.
- Victories of the Space Marines (Anthology) edited by Christian Dunn, "Runes," by Chris Wraight