The Picts were a tribe of people who lived in 2nd century Scotland. They were mainly hunters and farmers, who lived in peace. When their homeland was invaded by the Roman army, they became warriors in its defence.
The Picts adorned their faces with conspicuous designs, armed themselves with rudimentary spears, small round shields and dug traps and lived in roundhouses. The Romans referred to them as barbarians.
Every generation, a Pictish warrior entered the Devil's Cairn and fought the Eater of Light so that it couldn't break through into the universe. When the Romans arrived, the Picts released the Eater of Light with the hope that it would destroy them and be wounded enough for them to kill. (TV: The Eaters of Light)