The Cremated Key is a key that Heather Mason picks up in Silent Hill 3. Heather finds the key inside of the burn chamber in the Otherworld Brookhaven Hospital's underground Crematorium. Unless the difficulty level is set to Easy, the door to the burn chamber doesn't open unless Heather first solves the Crematorium Puzzle.
The key appears to have been burned inside the chamber, giving it its blackened appearance. It unlocks the door to the Day Room on the first floor of the Hospital.