Sometimes a unit will be almost entirely wiped out, with only a single member left. Such lone individuals must take a Leadership test at the start of each of their turns. If they pass they are filled with heroic fervour and continue to fight on single-handedly. If they fail they suffer an outbreak of common sense and begin to fall back.
All On Your Own (Last Man Standing)
Source: Core Rulebook 3E, p. 71
A unit reduced to a single surviving model must pass a Leadership test at the start of each of its turns or begin to fall back.