Character skins and Black Tar Keys in Mortal Shell 2 are highly useful special items. To obtain them, head to the Training Grounds, locate the entrance to the Citadel of Solace, and proceed down the long corridor deep within the fortress. You'll find a crossbow-wielding enemy at the entrance; defeat him to acquire the key.
How to Obtain Character Skins and Black Tar Keys in Mortal Shell 2