So did that actually just get sent back to the Mamodo world when their book was burned or was that some 4Kids Shadow Realm garbage. Which for some reason is the ONLY DUB I CAN FIND.
They were told that it was what happened, but the truth was that
they and all the other demons who didn't take part got their souls sent to purgatory and the winner decided whether to put them back into the demon world or to just kill them.
Adding to this, the anime kinda omits the progress for the sake of being able to put in filler, but there's some landmarks where the book indicates that there's 60 demons left, then 40, then finally 10.
Though strangely enough the mangaka made an interesting loophole where he had some of the demons from LAST MILLENIUM'S game turned into stone so that they could appear in this game.