The Cult of the Second Son is a Genestealer Cult native to the Hive World of Necromunda. Its genesis infestation was located in Hive Secundus, the second major hive city of the planet. The "Second Son" in the cult's name is a reference to its city of origin.
Hive Secundus was eventually destroyed by a radioactive bombardment that caused it to topple, but the cult was not destroyed.
Instead they took up residence in the vast subterranean warrens that lie beneath Necromunda's hives. The underhive gangs of Hive Primus have recently come into contact with the highly mutated cultists and the result has been internecine warfare.
On the infamous Hive World of Necromunda, the spire-like edifice of Hive Secundus was known for being rich in spirit and enlightenment as well as material wealth, more so even than its peer structure, Hive Primus, the capital of the world. Scholars sought to learn the mysteries of the Imperium from its extensive archives, and were so entranced by what they found they never left.
But its outward appearance was a sham, for Hive Secundus was under the sway of a vast and frighteningly influential gene-sect. When this fact finally came to light, the lord general of Hive Primus ordered the bombing of the rival metropolis with high-yield rad munitions -- so thorough was this bombardment that the entire hive city, built far taller and thinner than its counterparts, toppled onto its side with an immense crash that caused seismic disruptions for hundreds of kilometres around.
The entire area was declared quarantine extremis, and the denizens of Necromunda forbidden from even talking about it. Unfortunately for the hive lords, the bombing of the spire and its subsequent toppling did not prove enough to eradicate the cult that lurked at its heart.
So hardy and determined were the cultists that hundreds of thousands of them survived, and ventured out once more through the extensive tunnel networks that tendrilled from their old haunts. The underhive gangs of Hive Primus, their interest piqued by the very forbiddal of raids upon the fallen metropolis, entered those same tunnels.
The troglodytic creatures they found in there were mutated beyond all reason by the baleful emanations of the rad bombs -- and every bit as lethal as the Purestrain Genestealer from which they had been born.
- Codex: Genestealer Cults (8th Edition), pg. 17