A coven is a group of witches that have come together for a single purpose: apotheosis. This is a divine state of being in which a person becomes an idealized version of their own vision of perfection. This is done through a ritual that requires a circle of 9: eight witches and one sacrificial victim. It is performed over 12 hours, in which runes are carved into the victim's skin. Incantations activate the runes, cooking the victim from the inside as their aether (life energy) is extracted. The aether is absorbed by the witches while leaving the sacrifice a desiccated husk.
A witches own magic increases as they absorb life energy of victims. Their physical form also changes to reflect their inner self, making every transformation unique, as well as their abilities. After many years, they reach their apotheosis and transform into their new body. This godlike being is far more powerful than regular humans, and is biologically immortal. Witches can achieve this state sooner or later than others, depending on when they joined the coven and how powerful they were at the start.
As things are, a witch who has achieved this ultimate state has no use for the coven. They could easily kill the other witches, or simply leave to pursue their own plans. Alternatively, a witch could kill another member who they felt have gotten too powerful. How can I preserve the cooperation of coven and prevent the inevitable backstabbing and treachery so that all the witches can achieve their goal?