The boss died, the thing got destroyed and I left the cave. I think it took me 3 runs (and then one million lives).
The game 100% feels like a NES game up to the slightly rigid controls (not only not using the sword while jumping but also not being able to control your jump while you're using the curse item), it looks great, it sounds great and it's super fun to play. The difficulty is perfect with sequences of the appropriate length and it ends with a satisfying boss sequence.
Thanks for sharing!