Age of Empires II: Definitive Edition (PC) - Achievement - Relic Hoarder
Collect all Relics on the map in the fifth Suryavarman mission "Nirvanapada".
This achievement is fairly simple - Just gather all the Relics. You start with a few in your bases so be sure to grab and store them first of all. The remainder are scattered throughout the map and you can search and gather them or (and this isn't actually a bad alternative) you can allow your enemies to gather them. The reason this isn't a bad option is that they will all be in one or two places for you to gather. They will likely be captured by the Red or Blue enemy. The only Relic that you may not naturally come across is on a tiny island at the top right of the map so be sure to send a Monk in a Transport Ship there at some point.
The only danger to avoid is that the map is that the mission will end if you kill three enemies, so if you're destroying bases in search of Relics, be sure not to kill three enemies fully until you have your Relics.
Explore as much as you can and keep an eye on the map for any white dots representing Relics. Also, the more you explore enemy bases, the better grasp you will have on where Relics are as a Relic in a Monastery is represented by a flag on the Monastery showing that something is garrisoned inside.