The Black Marrow Key in Mortal Shell 2 is a very useful special item that can open special chests to obtain Shadow Skins. There are five in total. To collect all the Black Marrow Keys, you can acquire them one by one. The first Black Marrow Key can be purchased directly from the Hidden Merchant.
How to Obtain the Black Marrow Key in Mortal Shell 2
Mortal Shell 2 - Hidden Merchant - Black Marrow Key