Very nice atmosphere. I like that there is a progression in the game.
I have to mention that it's pretty annoying trying to pick up the mana drops, it's very hard to click on them for some reason, it just does not register or focuses on a slime that might be running behind the cursor at the same position. So I kinda had a lot of mana balls just lying on the ground and a bunch of slimes blocking the clicks :) I know that there is an item in the shop that you can buy, but maybe it's something that could be made available right from the start if the clicks can't be fixed. Also, the tutorial menu has a very sensitive scroll (I am using the touch bar).
Other than that a very solid game, good job!