the third series of merlin finished yesterday. 

I have watched every episode several times and am really puzzled by how much I love this show.

Some clues though:

morgana in series one, suspecting that she herself has magic, says to merlin:

'what if magic isnt something that you choose; what if it chooses you?'.

a painful, powerful secret; a devastating and hated scar across one's life.
this I understand.  
merlin has this powerful secret and it becomes his greatest strength:
a means of redemption no less.