A qalankhkai (plural: qalankhkan) was a Romulan Qowat Milat warrior who chose to serve as a "freeblade", binding her sword to a singular cause that she had judged worthy. Their criterion for worthiness was that it be a "lost cause".
As of 2399, half the Qowat Milat sisters on Vashti served as qalankhkan, attempting to keep the peace by patrolling roads and waterways, helping travelers, and defending Romulan and Human lives. That year, Elnor chose to bind himself to Jean-Luc Picard as a qalankhkai, to assist him on his mission to find and protect Soji Asha. (PIC: "Absolute Candor")
In the 3180s, J'Vini bound herself to Taglonen's cause of protecting the Abronians until they awoke and claimed their new homeworld with J'Vini taking dire action with the appearance of the Dark Matter Anomaly in 3190. Her actions drew the attention of the United Federation of Planets who sent Qowat Milat sister Gabrielle Burnham, her daughter Captain Michael Burnham and Lieutenant Sylvia Tilly to stop her. Burnham realized that a technical fault was keeping the Abronians from awakening from stasis and repaired their cryo system, allowing them to awaken and settle on their new homeworld, thus fulfilling J'Vini's oath. J'Vini subsequently allowed herself to be taken into custody for her crimes, and she was extradited to Ni'Var to face Qowat Milat justice. (DIS: "Choose to Live", "All Is Possible")
Once a qalankhkai was bound to a cause, only she could choose to be released from the obligation. (PIC: "The Impossible Box")