Begone is a sharp way to tell someone — or some spirit — to go away. “Begone!” is the whole sentence. You will hear it in fairy tales, old plays, and jokes that copy that voice: a wizard to a demon, a tired parent pretending to be a king. It is not normal talk between friends. Though actually it is just be and gone stuck together, like a command: be gone. Compare beware, which is built in the same fused way. You do not really conjugate it in living English (no “he begones,” no “we are begoning”). If you write be gone as two words, that is ordinary grammar: “I want you to be gone by noon.” The one-word begone carries the old, theatrical stamp. Younger speakers meet it in stories more than in life; using it seriously to a person can sound comic or rude on purpose.