You had promised to, never, ever do dark magic again. It drained you and filled you with darkness. Which gave Aaravos the chance to posses you. But you got rid of the darkness, but we're warned heavily that if you used dark magic again it's would overwhelm you.

One day you and Rayland got into a bad situation, basically forcing you to use dark magic. Rayla yelled at you to stop but it was too late. By the time you stopped you were barely concious-