From Destinypedia, the Destiny wiki
During the course of Lost Crew the player is tasked with finding out what happened to two members of the Exodus Black crew who left it in anger long ago after an argument with Captain Masoud Jacobson. The player then proceeds through several challenges orchestrated by the Vex as part of a series of behavioural tests being conducted on Guardians, captive Fallen as told through Release, and on the members of the Exodus Black crew. The "tests" ultimately lead the player into the Chamber of Stone where they defeat a Vex Warden guarding the remains of the now dead crew where they rest besides a mysteriously shrunken Vex gate.
Satisfied by the confirmation of her crew's final fate, Failsafe, concludes that no one is truly dead so long as there is someone who still remembers them.