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...
If you look at a few pages back when Roux speaks, underneath the runes were faint words. This was to denote that over time Frisk was adapting to the magic in the speech and slowly learning it. Now they've fully gotten used to it.