Lost Nightmare Achievement in Doom 3: BFG Edition
Complete the Lost Mission campaign on Nightmare
How to unlock Lost Nightmare
This is the third and final Nightmare achievement. You’re almost done. This campaign is farily short, consisting of only seven levels. It isn't much harder than the base game or RoE, and you are given the Soul Cube. Follow the basic tips for "Nightmare" and this shouldn’t be a problem.