Darkness
A rogue Reaper. That was something you didn't hear about every day. Shepard had finally found some time to investigate with Dr. Bryson about his discovery. Leviathan - a Reaper that he supposed went rogue and killed one of its own.
Unfortunately, Bryson was murdered by his assistant. Not intentionally though. He claimed to be under control. No doubt Leviathan had some involvement. Shepard needed to get to the bottom of this fast.
Whatever Leviathan was, Shepard had to find this thing. If it could kill Reapers, she needed its help. Rogue Reaper or not, Shepard was determined to find it and uncover the secrets it held.