The Nightingale Trinity (or The Nightingales) is a faction within the Thieves' Guild in The Elder Scrolls V: Skyrim. Most, even within the Thieves' Guild consider them to be myths or something meant to scare the young thieves.
All Nightingales take an oath to protect the Twilight Sepulcher against all threats. Once when Nocturnal accepts their souls into the Evergloam is one released from this oath. After death, until their service is fulfilled, fallen Nightingales become Nightingale Sentinels.
All this is seen more as a business transaction than anything religious. If the Nightingales do well by Nocturnal then the Guild is rewarded with their luck. Most see this oath as respecting Nocturnal as the Patron of Thieves and not as a God.
Gallus Desidenius - Deceased
Agent of Subterfuge - Nightingale Subterfuge
Once per day, for 30 seconds in the affected area, people and animals will attack anyone nearby.
Agent of Stealth - Shadowcloak of Nocturnal
Once per day, for up to 120 seconds, while sneaking the Nightingale will become invisible. Interacting with the world and attacking will break the spell. During this duration, the ability can be reactivated by simply sneaking again.
Agent of Strife - Nightingale Strife
Allows the user to instantly absorb 100 points of health from a target once per day.