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The demon lord was the first hero. I'm not sure of everything that happened in the novel happened to him but from the latest chapter (chapter 31), the hero is powerful and cruel. He looks similar to the demon lord.
I suspect that something happened, the party died and the world restarted with the first hero now being the demon lord. But the world needs a hero, bringing Tae into the world.
That's why the demon lord made a comment saying its not his companions' time to die yet in one of the earlier chapters. He's been through this before.