Mass Effect 3
This achievement belongs to a DLC for ME3, unveiling the mystery behind "The Leviathans". Once you completed "Priority: Palaven", your diary will be updated and there's no pressure whatsoever to start this quest line, meaning you can't miss nor fail it despite of how much you've progressed in the main storyline.
Contrary to Omega and Citadel DLC's, you are free to continue your exploration / main or secondary missions in between this DLC. Should take you around 2-3 hours to complete.
List of Missions:
*** Spoiler - click to reveal ***Citadel: Dr. Bryson
Citadel: Leviathan I
Leviathan: Find Garneau
Mahavid: Leviathan
Citadel: Leviathan II
Leviathan: Find Ann Bryson
Namakli: Leviathan
Citadel: Leviathan III
Leviathan: Scan Locations
Despoina: Leviathan