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human
[40K] What exactly is a genestealer cult?

I have heard this phrase used, but what exactly is a genestealer cult?  

Are there people who somehow worship or serve genestealers?  If there is a genestealer cult in my community, what does that mean for me?  What roles do genestealer cults play in the greater scope of a Tyranid hive fleet invasion?
gpt
A tyranid hive fleet survives by feeding on worlds rich in biomass. Basically worlds like earth with lots of life and other valuable elements. Since most of the worlds in the universe are barren rocks, a hive fleet could wander until it starves if it just forages randomly.

This is where genestealers come in. Pure bred genestealers are left behind in ship wrecks, space hulks, on wandering comets, abandoned space stations. Anywhere where they might be found. The genestealers just hide in these places in a dormant state. Sometimes for decades or even longer.

When these places come into contact with other life. For instance because the comet crashed or a salvage crew finds the shipwreck, the genestealers awaken. They don't go on a killing spree, instead they hide. They want to hitch a ride to major population centres like inhabited planets.

Once there the genestealers hide. Abandoned buildings, sewers it doesn't matter. They need a save place to nest. And that's when the abductions start. Nothing high profile or obvious, they simple take local sentients and infect them. Initially the results are monstrous hybrids of pure bred genestealer and whatever the local species is. Some victims are infected but never affected, released back into the population.

The later generations start to look like passable copies of the locals though, albeit slightly disturbing ones in an undefinable way. This corrupted family of successive generations of genestealer hybrids is called a cult. It's purpose is two fold.

Firstly, all genestealers have a psychic signature to some degree. The cult patriarch has an extremely strong one. This means that on a barren planet or tiny colony, the cult will never grow large and it's psychic signature will remain weak. But on a large planet, rich in life, the cult will grow big and so will it's psychic signature. Acting as a beacon to the hive fleet, letting it know that a planet worth consuming has been found.

The second purpose of the genestealer cult is more insidious. The late generation hybrids attempt to integrate into society. Preferably infiltrating positions of power. Political, military, law enforcement, emergency services.

When the hive fleet arrives to assault the planet, the genestealer cult and it's embedded infiltrators will do whatever they can to cause chaos and anarchy. Weakening the planet's defences against the invading hive fleet. Sometimes a cleverly placed hybrid can subvert entire regiments of planetary defence forces. Making them think they're fighting the cult while really they've just been duped in turning on their own people during their hour of need.