The Spectre of the Horse Riders

Handed down through many generations of village folk and their storytelling tradition in the area of Llanbedr Hall, the North Wales country estate, is the tale of the seven horse riders.
Each sighting has the same thread running through and similar descriptions of seven warriors on horseback usually presumed to be and described as 'medieval knights' charging their way with steely determination whist flailing their weapons.

Some years ago, after the manor house had recovered from its derelict state and was being used for events and activities, a group of dancers were practising in the former banqueting area of the hall when very gradually the sound of galloping horses seemed to be heard. Only a few people were alerted at first, but gradually more members turned their attention to the oncoming onslaught, rapidly becoming a total invasion of thunderous sound..
Everyone's attention automatically turned to the right wall of the room where all the huge windows were and through them saw, in the distance, the spectre of impending slaughter as the seven aggressive  horsemen rapidly galloped towards the windows.
The group froze on the spot. There was no alternative. The horsemen would crash through the windows and annihilate them.
But that did not happen - to the dancers' absolute amazement the horsemen glided through the windows, through the group of people, through the wall to the left, and disappeared towards the mountains.

Who were these horse riders -  from where and whence did they come?

The group never quite recovered from this experience and consequently made extensive enquiries amongst local people but  to their chagrin most of the locals were not in the least perturbed. They'd heard it all before - if not the same, definitely nearly the same.

 
An illustration from Alan Lee - The Mabinogion of Dark age Knight