Clarel de Chanson is a mage commander of the grey wardens manipulated and corrupted by Lord Livius Erimond on behlaf of Corypheus.
In order to enthrall the Grey Wardens to his will, Corypheus used Envy Demon to make every Warden in Orlais hear the Calling. Out of the imagined incoming Blight, the wardens asked Livius Erimond to help them fin eradicating the darkspawn. He persuaded Clarel to raise a demon army, so she could invade the Deep Roads and kill the old gods before they become archdemons.
She sacrifaced masses of her people to summon enslaved demons. Eventually she discovered Livius was manipulating her and turned against him. She redeemed herself by sacrifacing herself in mortal combat with Red Lyrium Dragon summoned by Erimond.