Very fun to play, and nice visuals. Controls feel good.
The game itself is really easy because of the poison wisps. Maybe you could have a limit to the wisps and always keep the most recent ones, or maybe have a time limit for them, because the poor boss got infected to death haha.
Also, hitting enemies could have more visual feedback, more "impact". But other than that specifically, the game has a lot of juice in the visual department, and the sword effect is hella cool. Good job!