You can check the minimum requirements in the Forgotten Knights Endgame Faction window by pressing Y:

1. Once in the Factions window, click on the Endgame Factions button.
   
2.  Click on "Harbinger Progress" on the Forgotten Knights.
   
3. In the main Forgotten Knights window, click on "Harbingers".
   
4. This window will show the minimum required corruption needed for a Harbinger to spawn.
   

Double check the Harbinger Crest box is checked, otherwise a Harbinger will not spawn.