Age of Empires II: Definitive Edition (PC) - Achievement - The Missionary

Collect all Relics in the first El Cid mission "Brother Against Brother".

This achievement is fairly straightforward. There aren't any time constraints - You just need to know where all the Relics are and capture them. The achievement will pop once the mission is completed. This video begins once I've already established a base. The map will be revealed to a large extent once you build a Market.
Four of the Relics are in the far West of the map, in a wooded area where some Priests are. They will join you once you discover them and their Monastery will become yours so you can deposit them there. There is a fifth Relic - If you have a look at your enemies Monastery, you will notice a flag on it, indicating that there is something garrisoned within - The final Relic. Once you destroy the enemies castle, he surrenders and becomes an ally. You can obtain the Relic beforehand or simply change your diplomatic status back to enemy or neutral after this event and destroy his Monastery after he has made peace with you.