To destroy the dark magic within a witch is to call it out and rid them of it's power before it takes over, which means they have to die. It requires the cosmic power of Mother Earth's four elements to drown and bury the power which is not as powerful enough to kill Cassie Blake.
Coffin, the symbol of Mother Earth, athame, elderberry tree branch, the witch with dark magic, Soil in order for the spell to work , the witch with dark magic must be willing to participate. The ritual is to be set in the woods and the coffin will act as their altar. A cloth with the symbol of mother earth on it should be placed on top of the coffin. The elderberry tree branch needs to be inside it.
The witch with dark magic would need to focus their energy and mind on the middle of the Mother Earth symbol where as the other witch shall scatter soil around the symbol while reciting the incantation "I call on you Mother Earth, element of the physical, may you purge her clean". Afterwards, the blood of the dark witch should be spilled onto the middle of the symbol, where the lines connect. The coffin is to be then opened, revealing the elderberry tree branch. Snap the branch in half after the dark witch hands you the branch, this shall cause the dark witch to be rendered unconscious. By this time, place the witch's body in the coffin and close it. Place the cloth above it in the position where the witch's head should be. Using the athame, plunge it onto the coffin (hitting the middle of the Symbol). Raise your own power and call upon the power of Mother Earth to bury the coffin. The buried witch will die and her Dark Magic will die with her.