Age of Empires II: Definitive Edition (PC) - Achievement - A Truly Holy Emperor
Convert the Cathedral in Milan without acquiring a single Villager in the third Barbarossa mission "Pope and Antipope".
This method involves a little sneaking, but isn't all that difficult. Start by sending your priests on the transport as close to the Cathedral as possible and then disembark. Send the transport back and try and bring the rest of your units if there's time, but if not, take your strongest as the 5th initial member on the boat.
As long as you don't attack or go to close to their military, they will not attack you. The first thing you need to do is get inside the city. Keep your troops close to the gates and wait for an opportunity. There is a relic just outside of the enemy base that they will send a priest to collect. There are also sheep you can take which can help. Purposely lose the sheep to the enemy and try and follow it in as they bring the sheep within their walls. They also occasionally send trade carts through the gates. Careful not to block the gates too much or it can stop the enemy using them as it will detect the path is blocked. If all this fails, units will occasionally use the gate and you should eventually be able to get in. The more troops you get in the better, but try and get all your priests in. Any other units are a bonus that will help defend to buy your priests time while they convert the cathedral. Careful to avoid enemy units - there is a mangonel fairly close to the route in the video as well as an enemy priest near the cathedral itself. Troops will be quick to react once you start converting. Just buy time and protect your priests.
17/01/2021 - An update has changed the AI of the computer slightly which may cause the enemy to have more troops within their walls than shown in this video. This is still a viable strategy to get the achievement, however greater care may need to be taken navigating your priests to the cathedral.