Use "embrace shadows" while attacking AND taking damage. You will become invisible (usually can't even see your character at all), and attacking won't make you visible. The enemies will not react to you, but will take damage. I used this method to defeat most of the bosses in the game (the game had too much combat for my tastes, so I'm grateful I discovered this bug). I just looked at it as being more of a badass assassin than the game world expected.
Note: I haven't tested if any other conditions need to be present for this to work. I discovered this first when fighting that vampire in a cave south of Verdistis and I was playing Survivor.
