Age of Empires II: Definitive Edition (PC) - Achievement - Throne Thief
Steal the Stone of Scone in the fourth Edward Longshanks mission "Toom Tabard".
This is a secondary objective, so can be missed. Shortly after you begin the level, a countdown timer will begin signifying when The Scottish Army (Blue) will attack you. There is a destroyed bridge to the North-West of your starting location that stops you reaching them. You must wait until the timer expires at which point the bridge will be repaired and you will be attacked.
Once you repel the attack, build up a force, cross the bridge and fight towards the most northern point on the map where Scone (Yellow) resides.
The quickest route is to stay as North as possible and cross the second bridge. If you approach from a more southern angle, you will have to go through a second camp of The Scottish Army across some shallow water.
Once you reach Scone, the Stone of Scone will be garrisoned in the Monastery and will be guarded by a small force including a few Monks. Dispose of them and destroy the Monastery, revealing the Stone of Scone. You must then pick it up with a Monk of your own and garrison it in one of your own Monasteries in order for the achievement to trigger.