Complex sentence: Marquette University Special Collections and University Archives serves as the archival repository for the three institutions of the "Black and Indian Mission office" with their archival records known collectively as the "Bureau of Catholic Indian Missions", which generated the bulk of the records.
Split into simpler sentences: Marquette University Special Collections and University Archives serves as the archival repository for the three institutions of the "Black and Indian Mission office". Their archival records comprise one collection known as the "Bureau of Catholic Indian Missions", which generated the bulk of the records.