Imagine No Redemption

[Inziladun]

Hm. Could this be an example of a true repentance, without death or destruction?

Melkor hated the Sea, for he could not subdue it. It is said that in the making of Arda he endeavoured to draw Ossë to his allegiance, promising to him all the realm and power of Ulmo, if he would serve him. So it was that long ago there arose great tumults in the sea that wrought ruin to the lands. But Uinen, at the prayer of Aulë, restrained Ossë and brought him before Ulmo; and he was pardoned and returned to his allegiance, to which he has remained faithful.

Valaquenta

There we have a servant of a good Vala switching sides to Melkor, then back again, apparently permanently, with no lasting penalty.
If Ossë could do it, you'd think Sauron could have managed it also, had he wanted it badly enough.