It had been four thousand years since the fateful Tournament of Power. Goku, though having an extended lifespan due to the God Ki that he had initially absorbed, was not immortal. Thus on a mid summer night, he finally passed on to the afterlife for a final time. He had explored the universes in his time, at least to a simple degree and fought the strongest fighters the universes could offer. He was for all intents and purposes, complete.
ChiChi had passed on long ago, and his children, and their children had long passed, opting out of absorbing God Ki, being much less of fighters than their fathers.
Vegeta had become God of Destruction in Beerus's place two hundred years prior, and spent much less time thinking about his rival Kakarot.
As such, Goku didn't really have much to do. Thanks to his training with Whis, he had learned a perfected version of his Instant Transmission, removing the need to lock onto and energy signature or focus.
Frieza had been Hakaied by Vegeta when he had challenged him after Vegeta's ascension.
As Goku thought about his old enemy Frieza, he thought of the commotion he had caused in Hell over the past two thousand years. Frankly, while Goku would miss the challenge the Emperor could bring, even he knew that enough is enough. Still, Hell wasn't without its noisy and troublesome occupants, so he would occasionally go there and knock some sense into the rowdy folks.
This time, he had gone into the deeper levels of Hell to investigate a problem.
You see, according to king Yemma, Hell wasn't just a dimension. It had multiple layers, each their own universes in themselves due to the massive amount of beings in it. Simply put, Hell was like an office building, and each floor was a universe, all held together by the Abyss, a transitionary realm that existed between everything.
The problem was, that one of the prisoners in the lowest level, which would be considered the basement of Hell, had escaped into the abyss and if he found his way up to the universe or heaven, he could very well destroy all of creation. This was in fact, Demigra. He had been greatly weakened by his captivity, but he was still a threat to the local universe.
Goku, who had beaten him before, was to go and beat him again, this time for good.
It took Goku a while to find him, as the Abyss worked in a different way to space and time. What could be a single step for one traveler, could very well be the distance between ten universes for another taking the exact same step.
When he found the Demon King, it wasn't in the way he thought he would find him. There was no fight. No thunderous battle. No cacophony of blows and magic. Just a withered husk of a man that simply looked at Goku helplessly.
When Goku approached him in the vast reed and black abyss, floating in the nothingness, Demigra plead his case.
"I don't want to fight you Goku. It's not like I would win regardless, and the gods would never listen to my warning, but perhaps you will" The demon Lord choked out between deep rasping breaths.
"What are you saying?" Goku asked, floating next to his old enemy.
"Good, you're not as deadest in your ignorance as the gods above. Listen close, and listen well, Saiyan" Demigra spoke, each word a toll on his resolve.
"I'm listening, but you'd better not try anything" Goku warned.
"Hahaha" Demigra laughed an amused laugh.
"Do you see me? I couldn't do anything if I tried. My magic barely works and my power has been permanently sapped. No. I only wish to fix what may doom us all" Demigra waved away Goku's concern with a shriveled and wrinkled hand.
"There is a wave of entropy spreading through the Omniverse. It has claimed countless universes already, and with every universe it swallows, it grows stronger. Not even the Omni King or the Super Dragon Balls could not stop it. Only you can stop this. It's why I allowed you to find me and waited for you here.
I knew Chronoa would send you to find me and re bind me, but you know better.
Take a look through my mind and see what I have seen. If you still wish to lock me away afterwards I will go without resistance. I just wish for you to at least know what you face" Demigra finished, floating around in front off Goku.
Gou stood silently for a moment. He knew Demigra was a schemer and likely had a secondary motive, but what that was he did not know. Still, he could not find lies in his words so he reached out with his mind and looked through Demigra's. It was full of evil and heinous deeds, but he found the truth in the Demoon's words. He saw in his memories him barely escaping the wave in quite a few universes. He saw the countless things he had researched on how to stop it, het even through all the research, he found no way to stop it. He saw the gods dismissing Demigra, and the way they thought they could defeat the wave, yet all their theories and powers were useless.
Goku eventually left his mind and returned to his own.
"I get why you came to me, and I understand your plan. If you're really trying to fix this with me you're going to have to come with me, or you can get locked away again" Goku said reaching out to him. From the untampered memories, he saw Demigra trying to warn the gods countless times, yet the gods did nothing. The Kais were too weak and even the Chronoa in every multiverse could do nothing.
Goku was his last and only hope, and he was willing to do anything in order to stop the annihilations of everything.
"I will follow you and help you in every way I can. You have power and abilities even you don't know about. I plan to use those to stop this coming doom" Demigra said, shaking Goku's hand.
"I believe you. Take me to the next universe that the wave will destroy. We have a lot to talk about" Goku said, requesting the Demon Lord's aid.
"Of course" Demigra said, waving his right hand and opening a swirling red circular tear in spacetime and reality.
"Through here is a multiverse on the brink of the wave, you will have to find a way to stop it from within" Demigra said as Goku flew through the portal without hesitation followed shortly after by Demigra himself.