Complex sentence: Returning late one night to Baku, he went underground, hidden away for months in his apartment-when neighbors thought he had gone to Moscow, that he wrote one of his most important novels- "Between Two Worlds" (meaning Iran and Russia).
Split into simpler sentences: Returning late one night to Baku, he went underground, and remained hidden away for months in his apartment-while neighbors thought he was still in Moscow. At that time he wrote one of his most important novels- "Between Two Worlds" (meaning Iran and Russia).