Out of food, water, and ammo, you (the last remaining of your group) leave your underground bunker out of desperation in search of salvation. You will start out weak with low sanity. Your bunker is littered with the corpses of those you once knew. Sanity has been tweaked to be more sensitive to light and any sanity-reducing events (darkness, enemies, etc.). Your lantern also drains more quickly. Use your lantern wisely. Not a jumpscare-fest.