The dark clouds gathering on the horizon were an ominous portent of the storm to come.
The ancient symbols carved into the stone were a mysterious portent of things to come.
The eerie silence in the haunted house was a foreboding portent of danger lurking within.
The sudden appearance of a black cat crossing their path was a strange portent of bad luck.
The sight of a shooting star was considered an auspicious portent for the future.