Morax are an ancient alien species who were imprisoned on Earth. They look like tendrils of mud but can possess humans - both the living and the dead - using their bodies as vessels.
When the Doctor and her friends visited Lancashire during the witch trials of the 1600s, they discovered that a Morax army that had once been imprisoned inside Pendle Hill for their war crimes had escaped. A sacred tree on the top of the hill was the lock keeping the Morax inside, and it had been broken when it was cut down by wealthy landowner Mistress, Becka Savage.
The Morax began reanimating the corpses of witches who had been killed in the trials. They also infected Becka, who was taken over by the Morax Queen. The queen wanted to infect King James I but thankfully, the Doctor and friends were able to fend her off using torches created from burning branches from the sacred tree.
Using a piece of the tree and a little modification from her sonic screwdriver, the Doctor was able to reactivate the security system and return most of the Morax to the prison – all but the queen. She didn’t want to go, but before the Doctor and her friends could do anything about it, she was set on fire and incinerated by King James.