Revenge
"The three of us were friends, but Austin was not like Marcus. He chose revenge and selfishness driven by his jealousy." She looked at Meredith and continued. "And I fear he did something worse to your father."
Meredith, Atlas, and Marcus looked at her with sadness hinted in their eyes. With a sigh, Gwen further explained to them what she meant.
"I know that he taught you, Meredith. And he is the most powerful fae. He could do amazing things if he wanted to, what more if he has the book and the crystals? I fear that he may have trapped your father somewhere like an unknown place with the help of a portal or something."
Meredith hadn't met her father yet, but knowing this, she grew worried for him.
"You think Austin would really do that?"
"A jealous man can do anything, Meredith," Marcus chimed in, letting out a sigh. "If he had gotten what he wanted after gaining our trust, I fear that what Gwen had said is right," he added, Atlas nodding at his statement as he agreed.