It happened 100 years ago. When the third annihilator master along with all the squads were steadily fighting for humanity and rapidly galloping through the ranks of the powerful races. Kane Schleider was the most trusted comrade of the master then. He was extremely powerful. He was not a squad leader.
He was his right-hand man. They fought every war together. They were and still are known as the strongest duo in the universe. Although both of them were higher stage S ranks, when they fought and synced their movements together, they could even bring down a middle-stage Minemdum realm monster. They kept on fighting together for humanity to prosper. But one day, Kane acquired a second soul power. It was known as the demon soul power. We don't exactly know the details about it.
However, we do know that the power within that soul was immense. Even the godly soul couldn't possibly compare to it. Since Kane got a new soul his old one had just vanished into thin air. But the new soul gave him unimaginable strength.
Although he was an S rank, once he activated his kindred powers he could reach the power of a lower-stage Minemdum realm. But before he managed to gain the strength of a Minemdum realm he couldn't use his kindred power at all. At first, he fought for humanity with this for 2 years. Within this time, he managed to unravel several mysteries shrouding the demon's soul. One of them was that, whether the power of the demon's soul responded to the user or not was up to the user, but up to the soul.
There have been some old tales about the demon's soul we managed to obtain from old races even before the galaxy war, but all we managed to learn was that there have been some cases of demon's souls being obtained throughout history.
However, those people remained without soul powers for the rest of their lives. For two years, our human race kept on defeating the main races.
But then, the worst thing possible happened. At one point in time, a host of the demon's soul will face a massive question in life. We call it the crisis of identity. The way they answer the question will decide the rest of their life's path. If you fail to answer, you will fall to the path of darkness with no return.
If you pass, you will continue on the right path. Unfortunately, Kane failed to answer correctly. He wanted to continue on the right path. But the demon's soul took over him and he eventually became a very evil creature. Although his exterior looked like that of a human's, his insides were full of evil and darkness.
This was the start of the darkest war.