The Ancient relic gives you +14 Public Order. It is obtained from "Adventure". To access the "Adventure", click on the map icon at the bottom right. In order to place this decoration, go to 'Build', and then go over to the brown box that says 'Storage for Decorative items.' This decoration should be there if you have completed the corresponding adventure. That can be placed only in the main castle. Is an unique decoration. Later on, if you don't need this item for better PO things (such as parks and castle gardens), you can sell it for a decent amount of money.