The reward is a yellow version of the boomerang spell. It will be available in other ways in the full game, but it's the only way to get it in this demo!
You don't really need it though, everything's doable without it.
Also, you can press Enter + Middle click to skip a minigame attempt if you want to retry / stop faster ^^
Well, the heart-shaped cursor does make it kinda difficult to see where your click is, so it ends up missing more often than not (even when it feels like it should be connecting...)
Out of the 3 minigames, I struggled with that one way more than the other two myself, since it took a solid 30 minutes of attempts to get it TwT
Maybe making the ball hitboxes a bit larger would help?
