What did the voice ask Marvel to do and how did it terrorize him?
The voice belongs to the Invisible Man. When he flees from the inn in Iping, he leaves his clothes behind. Mr. Thomas Marvel is a homeless man; when the Invisible Man comes across him, Marvel is sitting by the road, about a mile and half from Iping. He seems to be trying to decide which pair of charity boots he will wear.
The Invisible Man makes himself known by starting up a conversation about the boots with Marvel. However, he does not show himself. Exasperated that he cannot see who he is talking to, Marvel demands that the Invisible Man show himself. Marvel thinks that his brain has either been addled by too much drink or the disembodied voice is a figment of his imagination. He does not believe the Invisible Man is a human being until he is hit by some flint-stones the Invisible Man has thrown at him. The Invisible Man then proceeds to touch Marvel on the face, chest, and arm to prove that he is real.
He tells Marvel that he has chosen him as his special helper because he is an outcast like him; he wants Marvel to help him procure some clothes and lodging. Stressing that he will need Marvel's help for some other errands of his choosing, the Invisible Man proceeds to intimidate Marvel by implying that he will resort to violence if Marvel does not do his bidding or tries to betray him. Thus, Marvel is terrorized by a disembodied voice who is able to frighten him into compliance through threats of physical injury or death.