Achilles bathed in the River Styx and the waters of the damned turned his skin to iron. It is more than possible that he fought against some immortals who would no doubt consider Achilles beneath them but the immortals did not have impregnably skin, they bled gold ichor.

[quote=@Agent B52]
Explain how these Grimm got their name, show some of their most notorious acts and give a brief overview of what is known of them.
[/quote]

Those statements are incredibly ambiguous compared to what you apparently actually wanted. 'what is known of them' could mean virtually anything from what the people know about then, to what is true about them, to what legends say. Clearly I'm not the only one to think so because of all the entrees only a handful only included what you were actually looking for. I realize nothing can be done now but in future try to be a little bit clearer in what exactly it is you want us to write about.