When Arthur discovers that both his manservant and adopted sister practice magic, he gets in way over his head trying to keep the pair of idiots from being caught red-handed.

Arthur meant to reach out to him, check him for injuries, but halted in his tracks when, for the first time, he noticed that it was a small girl that was cradled in Merlin's arms. "Merlin, who is this?" Merlin's grip around the child tightened, as though he feared she'd be taken from him. "She's mine," he said hoarsely. "Her name is Aithusa."

When Aithusa is imprisoned and tormented by Sarrum, she cries out for the only one capable of saving her. [Prequel to The Darkest Dawn]