The Vanished Knight - The War of Six Crowns, #1

The entity living inside Callan's soul orphaned her at age eleven. By the time she's sixteen, it's ensured her being shunted from one foster family to another. Her thirteenth foster assignment should be routine. Except...it's not. A psycho in medieval armor kidnaps her and she ends up in a magical world. There, she accidentally discovers a secret her parents had kept until the day they died. Both actually came from this magical world, but left before Callan was born.
To cover their tracks, they'd lied about everything. Even who they really were. Driven to find out where she comes from, Callan's trapped in a race for life and death. Walking away isn't an option, but if she stays too long, the entity will find its next victim. In this world where secrets are sacrosanct and grudges are remembered, finding the truth will be near impossible. Especially when Callan has her own homicidal little secret to deal with. One with a taste for destroying her life.