Nice bossfight design, I enjoyed how the theme was employed and the art, especially in greyscale, works really well. The controls felt a bit clunky to me, I found the dash very hard to utilize well and would have liked to feel a bit more feedback on some of the player's movements. 

I wasn't clear either how the altar should be charged up initially so it took me a good while to even get the game started. A bit more tutorialization or just sound and animation to indicate when something has happened would elevate it a lot!