Uther Pendragon conquered and controlled the lands of Albion (England) by force.
When he dies and young Arthur had become a Supreme King (496 AD), many lords and kings saw an opportunity to get independence. Within 1 year, Arthur regained control.
In 497 AD, Supreme King Arthur ordered that all noble kids born in May of that year are to be put on a boat and drowned as a sacrifice to old gods. The law is obeyed and 13 kids are put to a boat and send to the sea to die. Soon after, the second rebellion has started. King Arthur won again.
After that, he officially accepts Christianity as an official religion in Albion (still, pagans are not persecuted).