After you’ve had your first encounter with the massive Cyclops, also known as the Krell Guardian, in Chronos: Before the Ashes, you’ll be wandering around finding a bunch of locked doors. These doors will impede your progress, but there’s a distinct key waiting for you behind a locked cabinet. There doesn’t appear to be any way to gain access to the cabinet through a key. To grab it, you need to shrink down.
If you appear inside the cabinet, you can see a small mirror at the top part of the cabinet. That mirror contains three distinct symbols that you can enter if you place it into the mirror that you have. Make sure to swap the primary mirror’s symbols to match these three, and then you can enter the cabinet. The first one will be an arrow pointing to the bottom right, the second will be an arrow pointing upwards, and the third symbol will be of a backward C. The first block, the downward arrow, is missing, but you can find it in the level.
When you rotate the symbols in front of your mirror to match those inside the cabinet, confirm them, and then step through the mirror. You’ll be transported inside of the cabinet, and you’ll be significantly smaller. Fight your way to the bottom of the cabinet, and you can push the master key to the floor outside the cabinet.
Now, return to the mirror inside of the cabinet to return to normal size, and then return to the cabinet. The key will be at the bottom of the floor, and you can now access the many locked doors throughout the level.