Hyndel fled the the Crossroads into Winterwatch Tower and joined the cult because he found their religious beliefs to be right. He asked his parents to join him there, but they refused because they were too attached to their land, according to Hyndel.
At the Crossroads, the Herald can approach Hyndel's father who will ask them for help. His wife, Hyndel's mother, is ill and Hyndel is the only one who can make a potion which will make her better. Hyndel will make the potion for the Herald on the spot.
If the Herald returns to visit him again, they can persuade him to go back to his family and leave the cult.
After telling him to return, Hyndel will appear inside his family's family home. His father will thank you for your help in convincing him to return. Hyndel can no longer be spoken too.