The Nightingales are the elite protectors of the Shrine of Nocturnal. In exchange for being given blessings by Nocturnal, they are bound to protect the Shrine in life as well as in death, until their debts to Nocturnal are paid.
Nightingales have always been members of the Thieves' Guild and were considered just legends by members of the Guild — a reputation the Nightingales themselves encourage — until recent events thrust them into the limelight. There are always three Nightingales at any one time.
Becoming a Nightingale will grant the player a Nightingale Armor Set that enhances Thief-related skills.