User blog comment:Aithusa07/List of Plotholes from Merlin/@comment-123.2.187.141-20150513150005/@comment-80.194.173.91-20180131171828

History/legend implies that, like Harry Potter mentions, At some point magic and mortals had one last stand off..... and humans won. It seems like this is similar. That a powerful advanced people couldn't be beaten by a mad man with a bunch of soldiers with swords. I do agree. He must have pursued them and slaughtered them like a real b****ard to have won the purge...