The Rising Mist — M.J. Evans

Perfect love casts out fear. 

Celestia, home to the Legion of the Unicorn, is a remarkable place where the power of love prevails over fear. Nick and Bethany, immortal humans, are unicorn riders. Along with other legion members, they and their unicorns, Lazari and Shema, rescue and protect animals on earth that are in danger.

After many successful trips to earth, Nick and Bethany must face a crisis closer to home. The evil unicorn, Hasbadana, has captured Nick's sisters. Urijah, the Lord of Celestia, sends Nick and Bethany to confront Hasbadana and bring the sisters home. Instead, Nick and Bethany are themselves imprisoned in a swirling mist of darkness, deep within the crumbling stone walls of Hasbadana's castle.

When Urijah willingly sacrifices himself to take the place of the humans, Nick is determined to find a way to free Urijah. He seeks the advice of Animara, the patriarch of the fairies. It will take the combined efforts of the Legion of the Unicorn and a new partnership with the animals of the Underground Kingdom to execute Animara's unconventional plan. Even then they don't know if it will succeed.