Sisters of Cydonia

The Sisters of Cydonia were a cult of techpriest assassins that were within the Adeptus Mechanicus and lived on their homeworld of Mars in the Cydonia Mensae.

They were noted to serve no authority beyond their unknown masters that lived in their own lands. One member of the Cydonian Sisterhood aided members of the Dark Mechanicum during the Horus Heresy at the time of the Martian civil war.

Source

 * Mechanicum (novel)