Thorns of the Black Rose

This is the fourth book in the Havelock Emerald series, where Lucas, Alex, and Celeste find themselves back in the realm of Valasar. They find themselves in a quest to not only slay a dragon, but a conspiracy to save one, as well.
When a lesser king and his two Sword Lords refuse to slay the dragon, Elendria, in order to steal her Star Fire, High King Manix declares them outlaws. When he orders that their three children are to be executed, Lucas, Alex, and Celeste become determined to rescue them, for one is to be a legendary Sword Lord, the other is to become the Lion Lord, and the third is destined to become the queen, Alicia the Black Rose.