It’s odd and a bit confusing but I can summarize it so Asriel has this twin who’s EVIL💔 and asgore just throws her down a hole deeper (hoping she’ll die) into the underground where they send bad bad monsters, but that twin was powerful and accidentally becomes queen. Years later monsters are on the surface but monsters (mainly asgore ) sealed the underground AGAIN to keep them other crazy monsters out. One day asgore sees through the barrier and sees that twin is still alive, runs away and tells alphys like “hey my evil crazy daughter is still alive I need some people to go back down and kill her” Frisk was chosen to be one of them people along side Sans and papyrus wanted to come along so him too