First thing I noticed, I loved the little intro sequence, both of the transition of the logo into the screen, as well as the actual intro cutscene. The art is very nice and I like the music too! The variety in sound effects is also really appreciated and really puts you in the shoes of the sword.
I also appreciate the nice touch to make the sword more golden as you progress through the game. A very neat touch!
I did notice a few things that I think might be worth pointing out. The game crashed two times, both times when the sword was stabbing the floor. The music would keep playing, but the game screen would stay frozen and the controls would cease working. For reference, I'm using a regular Google Chrome to play the game.
Considering that this game has many separate levels with time leaderboards for each one, I'd suggest letting players pick any of the levels from the get go, so that they don't have to go through each level again to get to the level they last left off. Alternatively, you could make a system to save progress. Since the crash happened to me twice, I had to start the game from the beginning due to the level select having the levels locked until you beat them.
I'd also suggest making a system to keep the cursor in the middle. Throughout my play session, I had to keep repositioning the cursor to the center of the screen every time I'd go for a new throw, to make sure the throw would be accurate.
Another suggestion is to put the Pause menu to be brought up with 'P' key as well, so that people using fullscreen don't have to exit fullscreen with ESC first before being able to use ESC to pause the game.
Other than that, this game feels quite polished. The UI looks super nice, and the leaderboard system in game is simply awesome. I really like that you're able to see so many scores per stage as well.
The level designs can be quite tricky and devilish as well. This was a nice challenge! I haven't finished it, but I'll do it some other day. I made it to level 9.
Really cool game!