Years of therapy.
No, but really, what do you think would make him reconsider? I mean, if he's at the top of his game, ruling the world- whychange? Does he feel he's hurting the little people? That he feels empty inside because he has no one to share his achievements with? He realizes no one likes him bc they only fear/hate him? Like once you figure that out, you can go from there
1st Idea) You need someone to defeat you. Someone powerful, who will give a good "The Reason You Suck" speech, or perhaps someone who reflects something you once tried to become (if you ever did). Someone to wake you from the nightmarish power lust you've driven yourself into (if you have one). If your darkness comes from power this is a possibility. You would, of course, just make this up but yea it's still an idea.
2nd Idea) Another idea to start is just to reflect. Look at this world you've conquered. Sitting on your throne/in your home/on the battlefield, Think about things. What reason do you have to try to control it? To destroy it? What of all the people in your life that you've befriended/helped/etc.? What if they refuse to bow/follow you? What of the ones that have? Will you slaughter them in fury or in royal disgust? What fuels your reign of terror and your desires of world domination?
Basically, since you know your char, you just need something good enough to make your character think and question himself.
A good example would be my char. I'm going to make a Redux of my char Blackblood after he obtained all the powers of Vampirism, Lycanthropy, Dragon Blood and the Daedric arts, driving him insane with power and bloodlust, then have an Argonian hero stop him in the height of his power, losing it in the proccess. No power = no super psycho. And then more stuff involving Elder Scroll shenans but you get the idea.
Basically you need your own Phoenix Wright to come in and say "Objection!!"
A good idea is to do all the Daedric quest but instead of finishing their quests you would fail them or choose the good ending such as destroying the Skull of Corruption, refuse to kill Silus, lure the priest of Arkay to Namira's feast but kill the cannibals instead, turn the Azura Star into Black Star and kill Logrolf instead of luring him to Molag-Balls. But if you have some or all Daedric artifacts you can cast them into the lava of the Aetherium Forge as a symbol that you are no more their champion as well as preventing them to seek another one to do their bidding and offering their artifacts as a reward.
The Vigilant's of Stendarr got their entire hall obliterated by literally TWO vampires during the attack... Does an order that weak really deserve resurrection? I have also attacked the Vigilant's before and eradicated them without any trouble at all.
If anything, I would forsake everything and fight against the Ebony Warrior and make sure we both die at the same exact time. I do this every build and it makes me happy.