There are 5 relics on the map you have to capture split into two locations. This is somewhat missable though!
4 of them are on the left side of the map, to reach them, simply get Line of Sight (LOS) by building an outpost at the corner forest. You will then gain some monks and a monastery under your control. Just collect the 4 relics there
the last one is a bit tricky because you cannot capture Alfonso before you capture destroy the monastery in Alfonso's town. The town itself isn't too tough. You don't even really need to build a TC yourself. However I did and went in with some battering rams to clear a path. This is because the towers will wreck your army if you're not careful.
Once Alfonso's Monastery is destroyed, pick up the relic and collect it. Then finish the mission and the achievement will pop in Post-game. (It took about 30 seconds longer for me for some reason, be aware)