This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Qowat Milat were an order of Romulan warrior nuns. They were an all-female group; while a male could train in their ways, he could never truly be one of them. The Qowat Milat were the most feared enemies of the Tal Shiar, and, by extension, the Zhat Vash. (PIC: "Absolute Candor")
Beliefs and practices
The primary teaching of the Qowat Milat was the Way of Absolute Candor, i.e. the total communication of emotion without filter between thought and word; this ran counter to everything that mainstream Romulans held dear. According to Zani, theirs was a "house of truth". Another of their sayings was "a promise is a prison."
Jean-Luc Picard described the Qowat Milat as "the most skilled single-combat fighters that I have ever seen". Indeed, they were sometimes mistaken for assassins. They wielded swords known as tan qalanq. When possible, before attacking they offered their opponents a chance to retreat with the phrase, "Please, (my) friend(s), choose to live."
The Qowat Milat could not be hired, though their assistance could be requested. A Qowat Milat sister could choose to bind her sword to a singular cause as a qalankhkai, or "freeblade", if she judged the cause worthy. The criterion for worthiness was that the cause be a lost one. (PIC: "Absolute Candor", "The Impossible Box", "Nepenthe") This criterion also extended to providing their services as Shalankhkai during the T'Kal-in-ket. (DIS: "Unification III")
In the early 2380s, the Qowat Milat provided invaluable assistance to Admiral Jean-Luc Picard during the Federation effort to rescue Romulans from the impending Romulan supernova. On Vashti alone, they helped to relocate over 250,000 refugees. At this time, they took in an orphan boy, Elnor. Although this was meant to be a temporary arrangement, he would grow up among them. After the Attack on Mars in 2385 led to the withdrawal of Federation support, Qowat Milat qalankhkan contributed to maintaining the peace on Vashti.
In 2399, Picard traveled to Vashti aboard La Sirena to beg the Qowat Milat's help for his quest to find and protect Soji Asha. At the urging of their leader Zani, Picard made his case to Elnor, who agreed to bind himself to the mission as a qalankhkai. (PIC: "Absolute Candor")
The Qowat Milat played an essential role in establishing trust between the Vulcans and Romulans in the early days following the reunification of their societies. They continued to facilitate healthy discourse between the two peoples through the Burn into the 32nd century. Only sisters of the Qowat Milat were permitted to act as shalankhkai for the process of T'Kal-in-ket. (DIS: "Unification III")