The Iron Sisters are female Shadowhunters that forge weapons for all other Nephilim. They reside in the Adamant Citadel, their heavily fortified and hidden base, and speak only, and occasionally, to female Shadowhunters.
Iron Sisters are warriors. Once accepted into the sisterhood, the members are marked with sacred runes, allowing them to shape pure adamas, making them the keepers and the only ones able to craft and handle the metal into tools that Nephilim need. The Sisters call upon the power of the angels to imbue their energy into the blades they make, during which they hear the angels whisper. Their blood is also apparently purified.
The Iron Sisters live and work in isolation. The Citadel is remote, reached only by Portal, and are only made aware of happenings outside the Citadel through the Clave. They are known to be brave and powerful warriors.