The Genestealers are a cult that are a hidden trump card of the Tyranids. They worship the Tyranids as savior gods, infecting the people with their genes to create a cult. The Patriarch is the most important creature of them all, because creates a neural link with all of the infected.

You were chosen by a strange figure, but you caught a glimpse of pale skin, purple ridges on it's forehead, and a third arm that has a small claw. You are taken by them, and you are put in a room where other humans and some hybrids are reproducing. You spot some first and second born hybrids, but they take you to a woman. Despite her alien looks, she beckons to you.

"Come to me. We bring salvation to you and your people. But we need numbers, and progenitors, both fathers and mothers. I have chosen you to father my next brood of hybrids. We serve the Emperor through the freedom."