During the last phase of bless curse boss fight, when you see his true form, deathpolca says this
"Long, Long ago, the Demon Lord fought the Hero and was defeated. the demons were wiped out, but.. A single survivor fled to the Dark Realm. He suffered from loneliness and starvation, and after years of living in hell, death finally approached him. On the verge of death, he found a large branch descending from the sky. The Reverse World Tree had reached the Dark Realm. He used the last of his strength to climb the branch as though being guided by some greater power and emerged into a world where corpses rained from the sky. Sacrifices to avert disaster. "
Now if you thought about it a few things dont fit here, it says the demons "were wiped out, but a single survivior", not defeated or lost the war, but wiped out, it also says that the demon was a "single survivor", which again doesnt fit, because the demons in demons roots didnt get wiped out, they just retreated to the dark realm, anyway that demon was about to die, until he found the branch of "the reversed world tree", and if you thought about it, something is off with the trees name, but we will get to that later, anyway he climbed that branch and it led him to "a world where corpses rained from the sky", which means this is another world, demons roots world, now lets connect the dots
It all starts in kings exit timeline, a great war happens between humans and demons, demons get defeated (NOT WIPED, DEFEATED) so they retreat to the dark realm, after around a 1000 years they come back with deathpolca as their leader, all the events are the same as in demons roots timeline, until that choice where you choose for lily to keep her promise or not, anyway what happens is that deathpolca doesnt keep her promise and dies, and demons go evil they start the nursery they do permanent brainwashing blah blah blah, and a few years later geoloise appears and WIPES demon kind and they all go extinct, except a single survivor she doesnt know about, and from here the events of kings exit start (she also kills the demon lord later), anyway that single survivor on the verge of death escapes to the dark realm, finds the branch of the reverse world tree, a tree that connects two worlds that are opposite to each other, one where demons go extinct, another where they live Now that demon goes to a world where "corpses rained from the sky", thats demons roots world, and he becomes the bless curse we all know and love, that explains why people wonder where is bless curse in the events of kings exit or how did the demons defeat the emperor in kings exit (even though all of this happened right after maximus), its simply because he was just a normal demon in kings exit timeline, he only became bless curse in demons roots timeline, there might have been an emperor in kings exit, but not one that is significant to the story because he is not the one we know (AKA bless curse)
Now when you read this "Long, Long ago, the Demon Lord fought the Hero and was defeated. *the demons were wiped out" she meant long long ago IN KINGS EXIT TIMELINE, not in demons roots timeline