Two sides of a moon

This is post (somewhat) apocalyptic world. David Collins was the one who is defending us against the evil entering our world in the form of mist through a portal.This mist influences and makes them their puppet. It can take the form of people and do things which may result in death, wars. It is Destruction itself. David had managed to somehow block the portal for a few months and had died in the process. But the gap is opening again. When he had closed the portal the evil magic had been weak but it is strengthening again. It is down to Haelly Collins to destroy the portal because David had told only his sister the magic and materials required to close it. But unknowing to most (though most of her brother’s forces suspect) the Lord of Destruction, himself has been influencing Haelly the most. She has dual personalities. So will her brother’s ghost and her lover help her to recover and save the world? Or will the Lord of Destruction steal her heart and very soul lead the world to its downfall?