The Cranes are an elite guard of female sidhe warriors sworn to obey and protect Prince Essus of the Unseelie Court. The number of Cranes was the same as the one of Queen's Ravens - twenty-seven. The Crane was Essus' symbol.
The Guard served the Prince until his death by assassination, having sworn their oath to. When he died, the Guard should by rights have been allowed to disperse, go back to private life or take an oath to a new master. Prince Cel, however, commandeered Essus' Guard wholesale, allowing none to retire, forcing all into his service without taking their oaths to him and adding them to those who served him under oath such as Siobhan.
When this breach was discovered by Princess Meredith, Queen Andais allowed all those who had not sworn fealty to her son to leave his service. Many, such as Biddy and Cathbodua, followed Meredith into exile in Los Angeles.