Yes the original Tyranid army ‘Genetealer Cults’ are back and they are bigger and sneakier than ever.
As the vanguard of the Tyranid collective, Genestealers hide on floating space hulks and stowing away on unsuspecting commercial scavenging space craft. When these salvage ships land on a populated planet the Genestealers slip off the space craft and slip in to the dark recesses of the spaceport. From these dark recesses the Genestealers hypnotize those at the lower edges of society, infecting them with the Genestealer DNA. The of spring of these thralls are the Hybrids that become the core of the insurgent army that will disrupt the defence forces just as the Tyranid main force arrives.
I have added a new Genestealer Cults Tactic page with loads of hints and tips.