Vivienne of Northumberland

Vivienne was a talented huntress and princess of Northumberland who spent some time in Camelot on behalf of her father. While there, she impressed Merlin with her wit and willpower, and he offered to make her his apprentice. She learnt well at Merlin's feet, and became quite the skilled mage. Unfortunately, though, her skill in magic cemented Merlin's desire for her--she was the first woman in centuries he could see as an equal, a peer. Vivienne didn't feel the same way, and rejected the mage's advances.

Eventually the time arrived for Vivienne to return to Northumberland, and Merlin offered to escort her. However, on the trip, Merlin received a vision that Arthur was under attack from Morgan Le Fay, and the two mages needed to return to Camelot to protect him. Vivienne was convinced this was another ploy, and thus, when they rested for the night, she cast a binding spell, and bound Merlin beneath the rocks in a cave of crystal.

Finally free, she returned to Camelot to bring word of Merlin's "death" to his friend, Arthur. But she found Camelot actually was under attack by dark fey aligned with Morgan. Guilty, Vivienne offered to take up Merlin's position as Arthur's court magician, and advisor. And while she was indeed skilled at magic, she lacked Merlin's political instincts--the man learnt the art from Caesar himself. She tried her best, but she was not in the same league as Ambrosius.

This was of the factors that lead to the decline of Camelot. Vivienne failed to see Lancelot's affair with Guinevere, and she accompanied him on his quest to France to get revenge, leaving the throne open for Arthur's traitorous son, Mordred, to seize the throne. She was there at the death of Arthur on the battlefield, where he was struck down by the blade of his own son. Vivienne used her magic to sustain him, and took Arthur to Avalon, where the skilled mages and healers there tried to save his life--only to discover that Mordred's blade was cursed, and that even the greatest magics of the age couldn't save Arthur.

And so Vivienne did what she had to to save her king. If the magic of this age couldn't save him, maybe the magic of the next could? And so she cast a great spell, that locked Avalon one second out of time--and there it has remained for the next 1500 years.

Also, in the process, she let so much time magic flow through her, she bonded with it and became the archmage of time.

But yeah, thanks to Vivienne, Camelot fell and the third-largest of the British Isles--and one of the greatest centres of post-Atlantean magic--has been locked out of time for centuries now. Thanks to this, the Normans were able to conquer southern Britain and create England. Furthermore, what would have been one of the greatest threats to the rise of the Veil Treaty--the great magicians of Avalon--were completely removed from the picture, allowing the Veil Treaty to cover Europe in the Veil, and the Holy Inquisition to nearly destroy the European magical traditions.

Now, Merlin did eventually escape his bindings, and came down upon Vivienne with the fury only an archmage could muster. But he quickly realized that if Vivienne died, Avalon would be lost forever. So he's reluctantly let her live, though exiled from the British Isles. She's spent most of her time in France and the Low Countries, finding champions and heroes who will help her in her quest to find redemption, and discover a way for her to reverse her spell and return both Avalon and Arthur to the world.