When he was a child, Mason was told the tales of Henry Parker, who was cursed to become Kohonta, a creature that bound to the forest and fed on unfortunate victims. Mason had believed them to just be a tribal legend but kept in mind of how to kill it.
In 2019, Mason finds his son Thomas with his girlfriend Barbara in the woods. The pair argue, as Barbara goes off. Mason reminded Thomas that he told him not to enter the woods, before they hear yelling and find the girl has been killed. Mason suspected it was because of the Kohonta but he withholds this from his son.
The next day, he meets the Winchesters who orders to stay away from the woods after a mysterious death.
Later on, he is found investigating in the woods where they confront him. Mason reveals details of the Kohonta and they reveal the supernatural to him, as well as reasons not to expose its existence to the public when he says the world should know. At that point, Thomas calls him and states he is going to kill the monster, despite Mason pleading to him. Mason then asks the Winchesters to help save his son from the monster, as Dean promises to help and Sam voices they don't know how to kill it but Mason says he knows. He then states a silver blade to the heart can kill the Kohonta and Sam reveals one on hand.
After finding the Parker cabin, Mason arrives to save his son from being killed and attacks the creature, as Dean carries Tom out to safety. The Kohonta bites Mason in the shoulder, and Sam shoots the monster. They fight and as Dean returns, Mason grabs the silver blade and goes outside, and Dean taunts the Kohonta to come at him. When it follows him out the door, Mason stabs it with the silver blade and it dissolves in a matter of seconds. Mason confirms that Tom is alive but unconscious.