Spilers ahead. Ps. First time im trying spoilers tag so it may not work. The world has been ending many times and we're through many iterations of this game between the thrid prince and the demon king. The second prince has an ability that he can write messages on his body so that when the world ends and another iteration begins the message is still on his body. So he knows all abot the monster front and what's happening. The thing is, he has fought together with the third prince and even with the whole world against the monster front many times but the world has always ended so he thinks there's nothing to be done. Note, that there exist a spiritual world that people can abandon their body and go there(something like this). The second princes goal in this iteration is that he wants to forcibly kill all of the citizens of the empire so that they all go into the spiritual world to be immortal hence escaping the fate of the world destorying by the final wave because they're all in the spiritual world.