Shadow Demon was a creature that haunted Caversham Hall. In 1807, Mannering Caversham let it into the Earth but trapped it in the glass of the house. In 1907, it was let loose again and Leela worked out how to reduce its rampage. Oliver Mortlake was determined to get it under his control but the Fourth Doctor stopped this by re-trapping it in the glass and then taking parts of the glass so it wouldn't be released again. (AUDIO: The Darkness of Glass)