Interesting idea and nice mechanics! I used wat more than 3 lives to beat him, but it was fun :) The idea of needing to charge the altar is pretty cool!

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!

Really awesome game! The visuals were super polished and the controls felt really nice. I have a few suggestions but they're pretty nitpicky:
Some of the time the boss would spawn directly on top of me, maybe I'm just super unlucky but this was pretty sad because it ended a lot of my runs. Maybe make him not do damage/attack until some period after he fully materializes?
Maybe make a neutral dash input go in the direction the player is facing (this is really subjective).
Super fun game and I can't believe you made this in such a short time!!!

That was cool! I like that you got a lot out of a small space by going vertical.