After you defeat Gwyn; the last boss, you are presented with a bonfire. Depending on what you do at this point, decides whether you get the "To Link the Fire" ending, or the "Dark Lord" ending, and their corresponding achievement.
If you light the bonfire, you will get the "To link the Fire" ending; If you decide to turn your back and walk away without lighting the bonfire, you will get the "Dark Lord" ending.
Each ending nets you a separate, 90 point achievement.
This specific achievement relates to the "Dark Lord" ending.