The child is cursed—Eryndor always thought. He has discerned that keeping Alex close meant death to everyone around him. At that time, he had to consider things as a patriarch of the clan rather than a father.
It didn't help that the woman he devoted his life to cheated on him, so the slight inclination he had to save Alex was overwhelmed by the sense of hatred.
Eryndor wondered if Alex was his child or not. The evidence was there, and Eryndor was gifted with a child, which was inhumane. Naturally, he would doubt.
At that time everyone deemed it right to erase his memories and seal his powers. And to be on the safe side, keep Alex imprisoned for the rest of his life.
Eryndor agreed to it since, his child or not, as the Patriarch of the Frostbate clan, he had to ensure the safety of his people first.
