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 it's rampage. Oliver Mortlake was determined to get it under his control but the Fourth Doctor stopped this by retrapping it in the glass and then taking parts of the glass so i wouldn't be released again. (AUDIO: The Darkness of Glass)