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This is Mari Lwyd (image cw'ed for skull)
In Welsh tradition, at christmas, Mari Lwyd will come around all the houses in the town with a bunch of mates and sing loudly that they want to enter your house
You are then obliged to sing back with reasons they can't come in and the argument continues until you run out of ideas and it then comes in with its mates and you have to give them ale and food
It's similar to wassailing where people go around door-to-door offering cups of mulled cider in return for gifts
And I guess trick-or-treating
But so much more metal