In The Mortuary Assistant, you’ll need to find two keys in order to unlock the game’s most difficult ending. The first key can be found in the basement, in a drawer to the left of the room in front of the PC. This key will unlock the storage room.
To find the second key, you’ll need to be on the lookout for a random event in the cremation room where a hatch appears. When you see the hatch, go through it and follow the doors until you reach a room with heads that you can interact with. Write down the numbers on the foreheads and then go back to the embalming room through the outside window counter. Here, you’ll find a darkened keyboard on the left unit. Interact with the keyboard and enter the numbers from the heads (197044). Use the hatch key to go to the basement, and then use the first key (the one from the storage room) to explore. Remember to burn the correct body to complete this task.
By following these steps, you’ll be able to find both keys and unlock the secret ending in The Mortuary Assistant. Good luck on your journey!