No Encore
Ensure Khiil Urd stays dead this time.
In order to unlock this achievement, you must complete Mission 41. In it, Harland will inform you that Thiil Urd, who was a suspect in the Demon Wars, has been awoken. Thoan's sent his forces to free him now that he is alive. It will be your job to eliminate Thiil Urd.
The game recommends you use Light-based weaponry as all enemies present are Water-based. Once you defeat Thiil, you'll be finished with the Mission, and the achievement will unlock.