To unlock this trophy, you need to kill all targets in the ''order'' menu, this includes all the zealots. The easiest way to achieve this is to play through the campaign, as a handful of targets are only able to be killed in the campaign. After you finish the campaign you can find the rest of the order members and assassinate them, by this time you should be able to locate everyone except for The Father and the zealots. Upon killing the final order member (Other than the zealots and The Father) all of the locations of the undiscovered zealots are revealed. You need to kill all of the zealots and then you'll have the total required amount of medallions, head to Hytham in your settlement and hand over all your medallions, this'll active a cutscene which will allow you to start a mission that takes place in Athelnay, travel to athelnay and a second cutscene will occur, which will lead to you defeating The Father in a cutscene. When the scene ends, Disorder of the Ancients will pop.