Mortal Shell 2 hides three families of keys behind its locked doors. The headline act: 8 Blackmarrow Keys open the eight Shade chests behind the lift in Marrow Keep, each holding a unique Shell skin — and there are exactly eight keys in one playthrough, so a complete set is guaranteed if you find them all. The Crypt Key lives in Fainweald near Baghead, and the Damp Key gates the Forgotten Crossbow inside the Flooded Village. Here is each one, where it is, and what it opens.
Eight keys per playthrough, opening the eight Shade chests behind the lift in Marrow Keep — each chest holds a unique Shell skin. Known sources include the secret DOOM E1M1 replica dungeon in the hub and a hidden room where shooting four chains behind a breakable wall reveals one. Optional maps sold by Merrick in Marrow Keep help you find the rest.
Found in the Fainweald area of the Desolate Peaks region. Find Baghead and exhaust his dialogue first, then check the ruins near his throne for a corpse holding the key. The locked crypt sits northwest of the One-Legged Wolf beacon, across from Baghead's throne.
Inside the Flooded Village dungeon northwest of the Blackridge Pass beacon: after the second elevator, look in a box by the house at the back left. It unlocks the dungeon's central building, where the Forgotten Crossbow sidearm waits on a table — see the sidearm list for the full route.
The most famous key source is also the best easter egg in the game: a hidden dungeon in the Marrow Keep hub area that replicates DOOM's E1M1 level — same layout, same enemies, same secrets — and pays out a Blackmarrow Key for clearing it. Between that, the four-chains secret room and the skins waiting behind the lift, key hunting doubles as the game's best optional content. None of it is missable: everything can be cleaned up in free-roam after the final boss, before New Game+.
The Damp Key route is one stop on the way to all 8 sidearms; for everything else behind the game's doors, see the beacon and region list and the beginner guide.