It's an interesting idea. I'm not a huge fan of chaotic random penalties though so I don't think it's for me.
Because without darkvision you automatically fail (and get hit by advantage/disadvantage for unseen attacker/target).
That was a few thousand years before Celts were around.Edit: It was also pretty slow; it was a couple of hundred miles across and took three thousand years to disappear, so it's on the order of a few miles in a lifetime.
Fun fact: Celts were originally central European, but the British Isles and Brittany were the only places Celtic culture survived the Romans.
It's an interesting idea. I'm not a huge fan of chaotic random penalties though so I don't think it's for me.