The crystal mountain (Forgotten Realms: The Empyrean Odyssey #3)

"Kaanyr Vhok backed up a step as the demons closed in. 'I have been to the angel's plane,' he said. 'I have spied on them. They are fools, and I know where they are weakest.' Having escaped from his gilded cage in Heaven into the claws of demons, the half-demon ruler of the Scoured Legion is finally in position to exact revenge on his angelic captors. Aliisza hissed. 'There are a hundred ways she can circumvent your desires while sticking to any sort of agreement she makes. Hells, she could agree to anything you want today and change her mind two days later. She's a demon. She can't be trusted.' An escaped prisoner from Heaven, the succubus Aliisza finds herself torn between her allegiances to her son, her lover, and the angel who set her free. Delve into the endless expanse of the Forgotten Realms multiverse in the final chapter of an epic adventure that crosses the planes of existence!" -- from publisher's web site.