@Siikalahna, All of them came there only recently and the battle occurred a week ago. It is a medieval-like time so the news are spread slowly and you cannot locate something as easy as it can be done in our times - you literally need to search for an enemy with your own two eyes on a huge territory
If I remember correctly from LN, these demons are someone who were buried under the corpses and they got out from there only when sensed humans nearby
@rberger, why should he? Because Reijy dived into this venture with demon king's subjugation even without asking Suimei's opinion? He has his own things to do and his own dreams\ambisions + he has relatives in the previous world, the same goes for his friends, and they just forgot about all what they left on the other side( or were "forced" to forget by Godness )
About his power - he isn't almighty in this world, all of his spells have less than a half of their firepower( or even less ) because legends and laws of this world are far too different from the ones they were build on