Unlike other worlds taken by Tyranid Hive Fleets, whose biomass is consumed before the Hive Fleet moves on to its next target, Tiamet has established itself on Ziaphoria while it creates a biomechanical edifice of unknown purpose that serves as some form of psychic resonator.
Ziaphoria, the repugnant and anomalous Jungle World claimed by Hive Fleet Tiamet, becomes the site of a disturbing new development in the curse of tyrannoforming, the hyper-accelerated biological process that is used to overcome and devour the prey worlds of the Hive Mind. There the conquering Hive Fleets have constructed vast psychic resonators of fleshy, encephalitic material -- some the size of mountains, some large enough to cover entire continents.
The world forms the end point of a vast pilgrimage of Genestealer Cultists from the nearby planet of Heinrich's March. Led by a blind prophet known as "the Conduit," they departed from a world plagued by the Dark Gods, plying the stars to eventually land upon Ziaphoria's pulsating crust.
Those who touched the corrupted earth with their bare flesh were instantly brought in thrall to it -- and convince their brethren to go back into space as missionaries, carrying the Creed of Tiamet to as many Imperial worlds as possible. They were the first of dozens of interstellar pilgrimages that sought out Ziaphoria, and in doing so, added to its power.
The Tyranids of Hive Fleet Tiamet defend the planet so ferociously it was declared Quarantine Extremis and abandoned entirely by the Imperium. Only the Deathwatch of the nearby Watch Fortress Haltmoat -- and Inquisitor Kryptman, who came out of his long exile to join them -- have any inkling of the threat posed by the immense psychic resonator of Tiamet. The theories they discuss long into the night are so wild, and the other threats facing the Imperium at this time so dire, that they are given little credence by the wider Inquisition.
- Codex: Genestealer Cults (8th Edition), pg. 39