Sona keeps her half sister at bay with her Mana. A set amount of mana is stored away and she tries not to use this stored mana to not risk Calla getting out. But when shes fighting her cracks expand and releases the mana when she fights. When using mana its hard for her to not use the stored mana when her main mana is being used. Once her main mana is gone, she has to use her stored mana. When the soted mana is tampered with, it increases the chances that Calla will be able to break free and gain control. Sona constantly holds her at bay and Calla fights against her mana when shes fighting.
When Calla gains control she decides to sit back and make Sona resort to her inner instincts, to lose her humanity and be consumed by the beast. Calla fuels some of Sonas powers when in this form but most the power is Sona`s own power that is unleashed in this form. Her scales darken to a black while her hair darkens to a dark purple, her marking pulse and glow purple. The cracks on her body are always expanded so the built up Mana can release, this released mana will act as powder that she can engite like a bomb causing it to explode. Can gather fallen mana to her mouth to chain it into a big attack. All Sona`s attacks are darkness beams or orbs she sprews out in quick sucession. Her mana crackles chaoitcally around her body, she acts like a beast when fighting and doesnt hesitate when attacking.
Calla can control her when her Mana is low or on crescent moons, when controled Sona is just a wild beast but will follow Calla`s command without a second thought. Calla has this control because of the scientist who worked on Sona. They merged the two sisters manas together so Sona`s body and mana would accept Calla`s orb more easily through transfer.