As you progress through the story, eventually you’ll be at a part where you need to drain queen Fay to power up the Tower Heart. You can only have one of Ghost or Dark Fay each playthrough, depending on whether or not you kill her before she returns to the Netherworld Gate after you have fully drained her. Continue draining to kill her for Ghost Fay, or leave her be for Dark Fay. Before you start this you can head back to the tower through the Netherworld Gate, which is close at hand. Simply make two saves and use one for Dark Fay, the other for Ghost Fay. This effects your Domination and Destruction %, make sure you take the correct decision for your main on your main save.