Arise! Arise my brothers in the name of the 4-armed Emperor! Arise and overthrow the oppressors!
Genestealer Cults can exist on any world, they seethe beneath the surface of the world, hiding away from those who would see them for what they are - creatures tainted by Xenos.
A Genestealer Cult is started when a Tyranid vanguard organism, called a genestealer, implants a poor victim. This victim's child is born horribly mutated, but its parents are blind to this deformity.
Over generations this taint spreads, and the less savoury looking members of the cult hide away whilst others integrate themselves into valuable roles in society. They plot as to how they'll overthrow the governments of their world, and make it ripe for their star gods, and the 4-armed Emperor to visit and bestow their blessings on their devout followers.
Once the Cult has reached the perfect opportunity they erupt from sewers, and other dark hiding places, and swiftly take an unprepared world by storm. This sends a psychic beacon to their star-gods to come to the world. Unfortunately for those in the Cult, this turns out to be the entirely alien race of the Tyranids, who then strip the planet of all biomass - including the Cult.
Within Warhammer 40k, the Genestealer Cult have a unique playstyle. Keeping lots of units hidden from their opponent during deployment, they have a range of tricks which allow them to appear right in the opponent's midst, and really reward a strategic commander.
Being largely a horde army, there's a huge variety of infantry and characters to pick from, and the models themselves are great fun to paint with all their flowing robes and industrial mining equipment providing a lot of textural variance.
