This is pretty fun! It gives you the feeling of a vampire slinking through the night as you have to sneak around and use the environment to your advantage.
I like the progression and gaining stats and power-ups. The speed and insta-kill both feel good to use and have different weaknesses they fill in. The speed falls off later though as you can increase your speed stats, leaving the insta-kill the best option I think since that lets you do something you can't normally. The dash power-up, while also giving you a new option, feels much weaker than the others. It's a bit like a weaker speed power-up. I think it'd be stronger if the dash felt more significant (distance), even if the short cooldown has to be increased.
In terms of stealth, the thing that worked the best is using the block and pushable objects to get closer to the enemies. The AI feels a bit erratic though, so it was hard to tell what the enemies were going to do, go, or look. The blocks do help greatly with dealing with this, but it's still very hard to deal with multiple enemies who are all covering each other's backs.
I like the idea of the long animation to kill, as it promotes a more stealthy style of gameplay, but I don't like the dash after killing. Whether after the long animation or after the insta-kill, it usually threw me into the light of another enemy. The dash after kills did not feel good for this reason.
I only got to the first boss for now, but I really enjoyed that fight (the lightbulb). It's a very cool arena and concept. The disappearing blocks and the giant lasers makes it a very fun fight.
Great job on this entry!