Complex sentence: Returning to Canewdon, he was invited to lead the local coven--the "Seven Witches of Canewdon"--which had existed since the mid fifteenth-century, and which he would continue to lead until disbanding it several years prior to his death.
Split into simpler sentences: Returning to Canewdon, he was invited to lead the local coven--the "Seven Witches of Canewdon"--which had existed since the mid fifteenth-century. He claimed that Pickingill disbanded the "Seven Witches of Canewdon" several years prior to his death.