The Ingressum

In an incident that little is known or understood about, Rome was under siege by an unknown force that appeared and then disappeared within a week.  They became known as Ingressum (latin for invaders)
The incident happened sometime between 37 b.c. and 22 b.c. and happened during the summer when Romans were all gathering for a festival in Antium when horns were heard in the distance.  Panic started to set in when the sounds of the horns seemed to grow louder and louder.
The mayor of the city sent a garrison of troops out towards the sound of the horns, and what they found were disturbing.  Around 1000 troops were marching directly towards the city.  Each one was reported to be wearing dark black armor with gold inlays.  Their skin was pale and their hair was reported to have been blonde.
When the garrison encountered the Ingressum, the Ingressum's horns got even louder and they started to sprint towards the garrison.  All but 10 of the troops were killed, while not a single Ingressum was laying dead.  The city was shut completely up, and they sent messengers to Rome to ask for help.
The Ingressum never directly attacked the city, but they did deflect several more attacks by the city's troops who did not manage to kill a single one of them.  They stayed outside the city for three days making loud sounds and blowing their horns before suddenly disappearing.  There were reports of sightings of The Ingressum all over Rome for the next century before the legends faded out.