Final answer:
The 'swift shadow' is a mysterious figure whose identity is not explicitly revealed.
Step-by-step explanation:
The 'swift shadow' that Hannah remembered racing into the dark trees the night before refers to a mysterious figure who moved quickly and silently. It can be inferred from the given passages that the figure could be a person, an animal, or even a supernatural presence.
For example, in one passage, a woman is described as spreading her wings like a shadow and flying. In another passage, a man is mentioned hiding in the sage and being associated with something terrible. Therefore, the identity of the 'swift shadow' remains unknown, leaving it open to interpretation.