According to dekudeals ( https://www.dekudeals.com/items/undertale), this matches the previous low for Switch and appears to be the consistent sale price at 33% off (Undertale is normally $15). This sale is likely due to the recent release of Undertale’s sequel’s second chapter (free).
In terms of content, these games are like indie Earthbound with bullet-hell mechanics and the option to spare all enemies.
Undertale ($10):
https://www.nintendo.com/games/de…le-switch/
Deltarune Chapters 1&2 (Free):
https://www.nintendo.com/games/de…-2-switch/