Origin 1: Sealed Overlord
You were previously a big bad with terrible and corrupting powers. Everything was going well until a group of pesky heroes managed to repress your darkened soul and 'save' you. That was a long time ago and you can't quite remember what it was that you did that got everybody soooo upset. Anyway, you've changed a bit since then, maybe you'd be more responsible with those powers now... if you could only remove those irritating seals.
Origin 2: Small God
One minute your in charge of a minor but essential celestial department, the next your obsolete. With your previous career path removed you've resigned yourself to treading the material plan in search of employment/greatness. Surely these mortals could benefit from your (minor) divinity?
Origin 3: Forgotten Legend
Your countless great deeds have faded over the generations, and also diminished in your own frail mind. Having reached the pinnacle of hero-dom and gained immortality you've sat and festered upon your crumbling throne. Slowly but surely, your powers ebbed away leaving you a tragic shadow of your former self. It's time to get back on the road and re-establish the legend of you.
Origin 4: Chosen Vessel
You were normal once. Now you are the puppet of a higher power, a conduit of its will. You can feel its presence burning inside you, changing you, manipulating you and it feels... wonderful. Lately you've been thinking you wish it would completely consume you.
Origin 5: Botched Summoning
Its bad enough that some mortal dragged you here, but their incompetence has meant you've not materialised in your normal (fearsome!) form. You'll reach your fully potential eventually but it will take a while. You may as well make a little mischief while you wait.