The Lawgiver was the free will of the Triskele.
After having caused great destruction through the universe and diminishing their species to a small number, the Triskele divided themselves into three subspecies to moderate their actions. The Engineers were the creative part, the Uncreators the destructive side, and the Lawgiver was the decision maker. Unlike the other two subspecies, there was only one Lawgiver. The Triskele could take no action without its approval.
When the Lawgiver was old and dying, the Engineers went to Earth in 1930 to find a new one. They could not choose an Engineer or Uncreator to replace it, as then the new one would be biassed towards those it had come from. When Peter Rathbone demanded the Triskele surrender to him and did not immediately agree, he shot and killed the Lawgiver.
This freed the Uncreators, who turned more bestial from centuries of suppressed rage. Before they could cause severe damage, Lord Tamworth fought and defeated the Uncreator Prime. Trying to mentally influence Rathbone to kill Tamworth, Peter was able to gain enough control to shoot the Uncreator Prime instead.