A major staple in The Exorcist franchise has been Max von Sydow's Father Lankester Merrin. The first film had him dying along with Father Karras after they banished Pazuzu. The second film then shared more insight into Merrin's past with the demon, while The Exorcist: Beginning detailed their first spiritual war in Africa.
Source:https://www.cbr.com/the-exorcist-believer-undoes-father-merrins-legacy-in-a-terrible-way/