In this story Jack came from a Mother who was a witch, he gained immense magical powers but there was a certain evil in him growing. Now he is considered a powerful warlock in his prime. However he abused his power too much and at night he transforms into a less intelligent monster. He is working with one of the players characters (Jason) and will appear to speak with Jason at certain points in the story.
In his usual form Jack is very cunning, intelligent and sly. Then when he becomes his beast at night his words don't flow correctly. I haven't really got a certain type of voice in mind for this character so let lose and have fun with it!