Solution:
This requires one play on the rewind game, & cannot be obtained in the original game.
Enter the trophy room. The door to the trophy room is different for each playthrough. It is either of the leftmost doors from the west wing on the top or bottom floor. Once inside, examine the box on the left with a green glint to it to obtain the dagger. Examine the meat locker to awaken the creature within. Use the dagger on the undead to obtain the cage key.