Board Thread:Show-Related Questions and Answers/@comment-25926288-20151112042649/@comment-27213286-20151123012525

My guess is that dark magic and light magic have different powers. Those that learned magic from Merlin's branch were taught the powers of the Light One and those who learned magic from Nimue's branch were taught powers of the Dark One. Just because two things have the same source doesn't mean they are the exact same. Dark magic and light magic are comlete opposites so it makes since they that would not be able to do the same things.