User blog comment:Edrea/Is Morgana more powerful than Merlin?/@comment-10341849-20130628150948/@comment-5674726-20130630000736

My guess is that the Triple Goddess somehow gave the power to her since Morgana was the last High Priestess, therefore needed more power because there weren't any other priests or priestesses anymore.

The thing is that the Triple Goddess gave Morgana a power boost of sorts by ensuring that she could not be killed by a mortal blade but it surely wouldn't reach the limits of the power and protection she could have been given. I suspect that Morgana was being set up as a martyr, to ensure that there could be no hope of peace between followers of the Old Religion and Camelot.