The Morax originated as sentient creatures, taking form of earth mud. They infect other beings by stinging them and traveling slowly through their body. They were also capable of reanimating corpses and taking control of living beings.
The Morax were vulnerable to the wood from the tree that acted as part of their prison which the Doctor described as "being like Semtex to Morax" which is why it had made such a good prison for them. Smoke from a torch created out of it was toxic to and repelled them while being set on fire with one of the torches could kill them. (TV: The Witchfinders)
In the 17th century they were accidentally released when Becka Savage chopped down the tree acting as part of the security system on their prison. This damaged it enough for the Morax Queen to infect Becka and use her to kill people in the guise of witch trials so that the Morax army could gain vessels for themselves. The deaths caught the attention of Team TARDIS who attempted to stop the Morax.
Armed with torches created from burning branches from the tree, the Doctor and her companions attempted to stop the queen from infecting King James with the Morax King. Using a piece of wood from the tree modified by her sonic screwdriver, the Doctor reactivated the security system, returning all but the queen to their prison. When the queen refused to go, she was incinerated by King James with a torch made from the tree. (TV: The Witchfinders)