Lone Survivor is an explorer perk that allows the player to survive longer than normal when travelling alone. When the explorer has no companions and drops to 0 Sanity, instead of an insanity event causing their death, a special event is triggered that fully refills the sanity bar and allows the character to continue.
Lone Survivor is an unpopular perk choice, as the scenario in which its benefits would kick in are rare and typically avoided at all cost.
This perk is a random reward option upon completing an expedition.