On the mission, Lost to Light, when you get to the door with the three relics needed to open it, go right from the door and you will eventually end up where the Shadow Thief strike takes place. Your ghost will say that he's receiving a fallen distress beacon and you can go into the ship by the grav lift. Once you're there, a timed mission begins to kill all the Taken in 10 minutes. There is a pretty annoying boss at the end of it who loves spamming those gas balls that mess with your vision. I haven't completed it yet since I just found it, but I'm curious if anyone has finished it yet or maybe knows what you get for doing it.