From the introduction
The Sisters of Silence are the militant arm of the Adeptus Astra Telepathica and are internally referred to as its Departmento Investigates. The Sisters of Silence have a long history (their organization predates the Horus Heresy). They are an all female order, in much the same way as the Adepta Sororitas; the major difference is that the sisters are all Blanks.
Recruited from Untouchable stock, the Sisters are anathema to psykers. Their very presence disrupts psychic activity and they are presumed immune to most forms of psychic assault. The main purpose of the Sisters of Silence is therefore to seek out and apprehend psykers. As warrior-investigators they can be involved in all aspects of the capture and transportation of psychic individuals and can almost always be found among the crew of the Black Ships.
Each Sister swears an oath of silence known as the Oath of Tranquility upon being nominated for Sisterhood; before this, when they are novices, they are allowed to converse normally. Amongst their other duties novices sometimes act as interpreters between senior Sisters and other agents of the Imperium. Full Sisters converse in their own sign-language, but there is at least one example of them being conversant in other forms of sign-communication, such as Astartes battle-signage.
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