literary a person or thing that signals or heralds a future event, change, etc; a precursor or forerunner
the cuckoo … that harbinger of summer and plenty — Edmund Burke
I am afraid I am not altogether a harbinger of good — Henry James
Middle English from Old French herbergere host, from herberge hostelry, of Germanic origin. Orig meaning someone who provided lodgings, later a person sent ahead to find accommodation, hence a herald; current senses date from the 16th cent