Caizher also learned that he needs to accomplish what his summoners need him to do, if not, he will be sent back to earth and he has a high probability of dying
As for the reason why Caizher needs to do that when the girl with an umbrella was powerful enough to send him into another realm, he doesn't know
Caizher is even mostly clueless about the realm he was in. He just knows that it was on the edge of the Dark Territory, a large part of the universe that is the turf of any creatures of darkness
Here, any non-dark creatures are rare and they are mostly visitors. There's not much hostility with the dark creatures to the other races. It's just that, dark creatures have quite a bad reputation and not many people would want to visit a place like Dark Territory that have the highest number of 'evil' people