Harbinger is the correct word.
It normally means a sign that shows that somthing is going to happen soon, often somthing bad.
- The November air stung my cheeks, a harbinger of winter.
- a harbinger of doom
- For this reason, Plato has often been cast as a kind of harbinger of the modern totalitarian state.
- As retailers know, Black Friday sales are not always an accurate harbinger of final holiday sales.
It also means a forerunner of something.
- These works were not yet opera but they were the most important harbinger of opera.
While Harbringer is not a word.