I know this might cause some controversy but it'd be awesome to be able to:
a) prevent the Damned One from fully awakening.
It's definite that he's become a lot more powerful but he likely hasn't attained the power to become some kinda god. Doing that requires a lot of time and training and effort, if I'm not wrong.
I'm quite sure that this process of fully transforming him tends to cause mental inbalances in the individual if it's rushed so the Black Ring has to tread with care.
After all, the process might desentisize you to violence and other types of "horrors" like betrayal, treachery, etc. and even harden your feelings but this also means you might develop mental issues like withdrawal from reality, various effects which impair your thinking and judgment abilities. Therefore, I doubt they'd dare to risk too much: what good is a brain-dead Damned One?
b) reconcile father and son.
If your character has enough strategies, tricks and various skills on your hand, I don't see why this isn't achievable.
Most games consider "all evil characters" to be "completely beyond redemption" yet for many morally suspect characters, they're just simply stuck in a self-destructive mode with no one to help them pull the brake. Basically, almost everyone around him is interested only in escalating the process and the individual is too screwed up to realise he's slowly killing himself in the process.
Therefore, it'd be interesting to be able to stop the process of awakening and even attempt to dissolve the hatred and other feelings within him.
Besides, if he awakens into an evil god, I want it to be caused by the player's actions and not forced by the developers. Otherwise, a confrontation between him and his father leading to possibly deaths of both and many others, feels very fake. It's like: what's the point of all these people dying? Surely there would've been another way to bring some closure to the story?
I know this proposed ending sounds impossible but then again, that's the fun in having an open-ended game, right?
Besides, I tire of all those games where you're forced into a confrontation without being able to resolve anything peacefully: by the time the game has ended, half the cast has died and it all feels so pointless. It's like: if the "evil boss" could've been convinced to give up arms and even perform reparations to atone for his deeds, then was killing him the right thing to do?
Finally: this is the reason why you have negotiators when dealing with bomb threats, hostage situations, etc. You don't just send out the SWAT team and splatter everyone. You at least make an attempt to extract all hostages before doing something really rash.