Yea sure, alright.
So John Blackwell came back to Chance Harbor because Cassie had apparently called out to him when she activated the medallion. At first she thought he was there for her, when he really came back for the medallion. But, as the episodes wore on, he was actually looking out for Cassie, but not the rest of her circle. He still has magic, but not enough to defend Cassie if the circle gets broken.
In the most recent ep, there was apparently a curse placed upon the Blake's and the Conant's so that they could never be together, and if they had, then someone in their circle would die. Cassie had gotten the info from Jane, who, unbeknownst to both, John was using some of his magic to influence Jane's speech to control what she said. At the end of the ep, it is confirmed that John still has a bit of magic, and that he is manipulating his daughter and the rest of the circle because when Amelia and Ethan were in love, it was so strong that they wanted to leave the circle. Hence why John made an elixir to make them forget how they felt about eachother, although it only worked on Adam. He did this because he needed the circle to stay intact so they could defend themselves because he doesn't think Cassie can defend herself with her dark magic if the hunters come after her.
) 
Thank you James 