For the mission do I have to fuse the four other ones to get the Justiciar or do I have to fuse the Justiciar with the 3 other ones to get the Relickeeper to complete the mission?
You don't need to do the entire fusion, just feed the 4 champions shown below the Justicar into the fusion to get the Justicar. You can use him for the Relickeeper fusion later if you want.