The day Monster kind was sealed into Mt. Ebott, the mages added a little extra insurance. A curse.
Human SOULS may pass. But Human forms may not.
Young Frisk must navigate the dangers under the mountain, a little less human than before...
@Gold Karma Dragon:
Papyrus is really fun to characterize since he's so quirky. He's sort of a Robin Hood figure in the Underground (as will be more apparent in future pages) so a lot of monsters love him. He's completely oblivious to it, naturally.