He was previously a member of the Vigilant of Stendarr years ago, alongside Isran. Dissatisfied with the order, they both left to form their own partnership, but it didn't last long before Celann began to disagree with some of Isran's methods.
Even if Celann is appointed steward of a homestead prior to Kindred Judgment, he will storm Castle Volkihar during the last quest of the main story. Upon completion, assuming he survives, he will return to the homestead he was appointed to and resume his steward duties.
It is possible for Celann to die during the vampire attack to Fort Dawnguard after the Dragonborn refuses Harkon's offer and returns there as he's not marked as essential.