Read to part 104:
This is a name used by the original Merlin when he went to Camelot to serve under Arthur. He called himself Emrys.
In my story the northerners pride themselves by the fact that they know this name. They see it as a sign that they know the Merlin family better than most people. So whenever they see a Merlin worthy of the name that the first Merlin used, they give it to them and name them as their protector.