Truly the Dark Souls of flash games :P
The core mechanic is pretty good. I found it entertaining to stealth around enemies and slay them.
However, it needs a bit more polish. Most enemies can be thwarted (read: completely blindsided) by utilizing the depth plane to your advantage. Some behaviors and sprites are a little janky, could use some more work.
I found the boss a bit jarring in difficulty. Compare to all the stealth mechanics at play in the earlier levels, the boss gave us an opportunity to use none of them. Instead, what ensued was a "kill you before you kill me" brawl between a glass cannon and a tank.
Fun concept. Cool music. Decent atmosphere. Needs work.
Oh yes, and the controls.. very awkward placement for the three keys to use. Might want to consider placing them closer together?