Geth Heretics are a small faction of geth that abandoned own race and planet under influence of the Reapers. They worship the Reapers as deities and serve them by invading planets in the galaxy, building observatory bases and lab stations.
However the Reapers don't care about their devotion, but obidience. They under leadership of Saren Arterius, attacked Eden Prime and the Citadel. After the former spectre's death they designed a virus that brainwashes all geth to join them and start to worship the Reapers as they do.