Spoilers ahead
This is the Redemption trophy, unlocked by acquiring the less conspicuous ending which is to save D6.
There are a variety of interactions and choices to make throughout Last Light that earn you morality points. If enough points are earned by the end of the game, you will unlock an alternate ending. Many of these morality points are simple however, many can be overlooked if you rush through the game. A flash of light will indicate the earning of a morality point when a requirement is met.
Some tips when approaching this trophy are to
- Play the instruments you find throughout the game such as the guitar and ukulele.
- Spare lives including those of Pavel, Lesnitsky and the Bear. This also includes avoiding killing soldiers, although you are still allowed to stun them.
- Listen to all conversations whether they be between enemy soldiers or between a few soldiers standing off to the side of your actual destination. I would advise to use subtitles seeing as most conversations that award a morality point seem to have subtitles appear on the screen while conversations that do not award any morality points usually do not.
- Exploration is another key element. There are hidden areas throughout the Metro that when explored, earns you a morality point. Additionally, some of these hidden areas will contain an ammo cache or something similar that when opened will award the point rather than just exploring the area.
- Another set of morality points is awarded by simply staying close to certain allies at certain points in the game such as staying near Pavel when he has the torch.
I would recommend to bundle this trophy with other miscellaneous ones such as reuniting the child with his lost teddy bear and saving the bear in the forest seeing as these acts do in fact earn you morality points. Also, move slowly and stealthily, avoiding killing during most of this playthrough If the morality requirement is met, this trophy will unlock when a certain someone prevents Artyom from destroying D6.
Good luck and happy hunting.