From Destinypedia, the Destiny wiki
Release is an Adventure located on Nessus. It is unlocked in tandem with all Adventures on Nessus after completion of the mission Six, and is closely related to the adventures Lost Crew and Hack the Planet.
During the course of Release the player discovers the Vex have been conducting behavioural experiments on the Fallen they have taken prisoner, whereby the Vex promise to release the Fallen only if they fight each other to the death. Demonstrating that Fallen hierarchies are bound together by more than just fear of their Captains, Barons and Kells, but are held together with actual and genuine loyalty as well, not a single Fallen attacks one another and they all wait patiently to die of old age, imprisoned by the Vex. So they would remain until the player, at the behest of Failsafe, enters the subterranean prison, murders the Vex, and releases the Fallen, who rather than being grateful attack the guardian, forcing the player to kill them too. Eventually the player proceeds far enough into the caves that Vex Wardens are sent to stop them. The player defeats these, and then learns that the Vex are gathering far more Fallen in more prisons, conducting research and experiments on them for an as yet unknown purpose. Failsafe's decade long imprisonment by the Vex during Hack the Planet, may be the cause of her sympathy with the Fallen captives when she says "imprisonment by the Vex is not a fate I would wish on anyone", and the trauma she experienced during her own captivity, and when witnessing the experiments conducted on the Exodus Black crew, may be why she is so insistent that the player's killing the Fallen is better than their remaining in Vex captivity.