This game is very hard, but it's generally the good kind of hard. The kind where you need to know the path you're going to take and the timing of when to jump or take a swing at an enemy. However, I feel like a game like this could benefit from having tighter controls. For example, I don't mind having an attack animation take some time and stop the player from running, but the time delay between the mouse click and the attack actually hitting makes the attack options unwieldy in the not-so-good sort of way.
The limited visibility lends itself well to the dark atmosphere of the game. There's just enough light that you can kind of see where you're going, but you'll still need to have a path memorized if you want to clear a level before the spreading corruption catches up to you.
Other people have mentioned checkpoints already, but yes, please add them to the game. Checkpoints in the right places are crucial for a game like this. The game demands many, many retries of each section for the player to learn the layout of the terrain and the placement of enemies, and to do that, players need to be able to quickly get back to the area they're trying to beat.
Lastly, when I'm getting off the broom, it sometimes gets stuck at some point in the animation and it completely prevents me from jumping. Lost many times from this, sadly.
But overall, I think this has the potential to be a most excellent game, and I would love to see a more developed version of this, if and/or when that happens.