She left
Disclaimer: I don't own Charmed.
I can still remember why she left; I'd misused the trust she had in me. I had tried to have her bare my heir for the next source and it back fired on me. Her sisters and that damnable white lighter figured out my plans and told her of them; I'd have Phoebe and my son by now if it hadn't been for them. I may be a demon, but I was never totally evil; I, like Cole, love Phoebe and always will. I just happened to lose sight of making sure that both were taken well care of, that should have been my main focus at the time; too bad I didn't see it like that. I had wanted to get rid of her sisters too, stupid move on my part I'll admit, the sisters have a stronger bond than the triad.
So here I sit on my throne, along, and wishing she'd never left me.