I thought Achilles killed the guy with the impregnable skin? Or was there more than one running around in Greek Myth?

EDIT:

[quote=@Lugubrious]
Now I kinda wanna know what you thought about Black Mercy.  I take it that it's because I made the Grimm in question, the one that's ideally legendary and terrifying, pretty much a pushover and only scary for its implications.  I knew that I was probably going the wrong route, but after reading a bunch of Lovecraft I couldn't resist the cosmic horror story route.
[/quote]

Again while I liked the idea my main gripe with it was that it just wasn't really clear how much was known about Black Mercy. How would it even have gotten the name when in the very same piece it stated that it has yet to be discovered by man/faunus?