The Crypt Key in Mortal Shell 2 is a very important special key in the game. To obtain the Crypt Key, you can first teleport to the outskirts of Noche, retrieve the Cemetery Key (the quest is likely related to the Sackhead side quest), and then head to Solitary Peak, where there is a Sackhead side quest.
How to Obtain the Crypt Key in Mortal Shell 2