Loved pixel art, atmosphere, and sound design of this game. Reiterating from a previous comment on another game that it seems the most fun game designs are the simplest ones, and strumming this one indeed felt like playing an instrument (almost independently from the varied sounds from destroying enemies, but that certainly helped too). The style and simplicity make Undertaken memorable. An impressive solo effort.
If I had to suggest any improvements, it would be a more explicit loss condition (I originally thought the idea was to ensure none of the enemies hit the grim reaper, but I'm not sure? It seems to rather be letting an enemy pass without being destroyed). Liked that additional enemies appeared, and the way to defeat them required more effort in addition to worrying about the others. Challenging stuff.
Fantastic work on this one. Very polished entry.