Others.
[Wait, she wouldn't know what those were. Recent threats, you know?]
People who have monster souls - they lose their minds and claim to be heroes who died a long time ago - and try and take down the four kingdoms, shattering the crystals to kill everybody. At least, that's what I've heard. The only Others I've met were those guys running a hotel and the group who tried to take this crystal.
[Yeah, he just gave an info dump. What are you gonna do about it, Light?]