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...
For the purpose of the comic, Grillby is a powerful Boss Monster that was originally content to just run a restaurant with his family- but due to the dangers of the Soulless stepped up to role of leader.