Jeff 'Joker' Moreau wasn't the type to brag. Which is why no one knew about it when he found the six crystals that gave him absolute power over the universe. He found them, by sheer accident, during the Collector Base mission, where they had been used as a power source for the matter conversion process.
Thankfully, no one had noticed the hobbling, crippled human pilot as he snuck over as best he could, stole the gems out of the device, became God, and left. He could feel EVERYTHING, and was aware of all the versions of how the final battle would go. He could sense the Reapers approach, could sense each of the worries of his crewmates, as well as the conflicted feelings of his commander, Jane Shepard. She hadn't romanced anyone after her boyfriend, Kaiden Alenko, had died on Virmire, and was thankfully holding up well. He could sense the Illusive Man's plans, and realized that he was becoming indoctrinated without even knowing it. It was slight, but if he kept up his plans then he would become just as much of a puppet as Saren was.
Too scared of his new power, he made no changes save for curing the disease that made his bones as brittle as movie glass. He would have someone invent a cure for it later. He reached out with the mind gem, watching as everyone fought. When someone was in danger, he willed them to become 'aware' of that danger, taking it out.
Finally, the human Reaper prototype was destroyed, Jane blowing off the Illusive Man's desires and blowing the base to smithereens. They returned home... only for Shepard to be arrested for some of her earlier actions, which resulted in a great many deaths in an attempt to stave off the Reapers.
Joker, outraged, turned his mind to the gems. He made a necklace out of them, one he wore at all times on an unbreakable chain. As the trial went on, things clearly weren't going in Shepard's favor. When the verdict was about to be read, his knew cosmic awareness telling him that it would be 'guilty,' Joker froze time.
Then he realized that he had frozen time.
"I... whoah..." he said, realizing what he did. He looked around, seeing vehicles outside frozen in midair. He looked back at the TV... and an idea occurred to him.
Making some 'adjustments,' he restarted time. Jane Shepard was cleared of all charges. And everyone was glad.
Especially Joker.
Soon after that, the Reaper invasion started. Joker's senses told him of every possibility of how it could go. And he didn't like any of them.
With his confidence boosted, he began making changes. Reapers and Husks of all types on every world suddenly became unable to kill anyone. They remained dangerous opponents, enough for the Krogan to respect them, but everyone else's competance went up in the meantime. Mordin was able to cure the Genophage without dying. He even cured Thane, the Drell returning to action. Legion was able to evolve the Geth without sacrificing himself. EDI... well, he didn't change how they met. He had become fond of the AI and having her get a body, especially one that gorgeous...
Anyway. The Citadel did what everyone wanted it to do, thanks to Joker's adjustments. Cerberus was put down for good, the Reapers were all destroyed WITHOUT sacrificing the Geth or EDI or any other AI, everybody lived. Shepard was acknowledged as a hero for all time, the Andromeda Initiative was dropped as not necessary, and absolutely goddamn nobody in the galaxy questioned why the super-dangerous Reapers didn't kill anyone in their invasion.
Which left Joker with some decisions to make.
"Okay, so... The universe is mine now," he said, looking at the gems in a mirror. "Lots to think about... I could start helping people. Be all subtle and stuff. I could just go on a fucking spree, live out every fantasy I've ever had... Hell, I could even make myself king of the universe." He chuckled, sitting back on a couch and relaxing.
"So much to think about..." he said softly.