This page of the Age of Empires 4 guide answers the question: what are Relics? We explain what you can use Relics for and how to acquire them.
Relics are special items that can only be picked up by religious units after your civilization reaches Castle Age.
The Holy Roman Empire and the Delhi Sultanate gain access to religious units earlier than others. Despite it, they still can't acquire Relics until they reach Castle Age.
When you find a Relic, its location is marked on the map with a golden artifact icon.
After finding a Relic and fulfilling the necessary requirements, send a religious unit to take it, along with some soldiers for protection. Select the religious unit and right-click on the Relic. Then right-click on any religious facility to place the Relic in it.
Each Relic placed in a religious building boosts its gold production. Some religious buildings can only hold 3 Relics at a time. You can check the maximum number of Relics in the building information.
Relics have a secondary use as well. Religious units holding Relics can use the Conversion skill, allowing them to take over an enemy unit.
The enemy will most likely quickly realise what you're trying to do. Your religious units can't fight back and aren't very durable. Therefore, they should be constantly protected by armed soldiers.
Unlike typical monks, the Warrior Monks available to the Rus civilization are very fast and have more health.