User blog comment:Edrea/Is Morgana more powerful than Merlin?/@comment-20237552-20130919223208/@comment-5674726-20130920000738

I don't think that not being killed by mortal blades was a High Priestess trait - Morgana killed Morgause with a dagger, and would have died from a wound caused by a mortal blade in the Season Four finale, had Aithusa not healed her. I'd say that it was exclusive to Morgana and I have two theories.

The first is that the Triple Goddess gave Morgana limited invulnerability to make her overconfident and lead her to her death, knowing that the slaying of the last High Priestess would ensure that there would be no merging of the Old Ways and the New. Those who followed the Old Religion may have become an underground movement, keeping their distance from others.

The second is that Morgana's healing bracelet was responsible. It may have worked by attracting healing magic when needed - Morgause when Morgana was poisoned, Merlin's when she was dying of a head injury and Aithusa's when she was dying of a stab wound - and then she was more or less immune to the same cause of injury from them on, as the bracelet stored the magic, in a sense, until it was needed again. This one is a stretch and the only thing I can really base this on is the way Morgana was knocked unconscious multiple times after Merlin healed her broken skull in The Crystal Cave but always recovered.