To gain access to the final boss's lair, you will need to purchase a map from the under city area in Alluris. A random event in town has a guard question you about the area, but you actually cant get in from the city (at least until you've gone to the city directly from there once in that playthrough.). From the main road enter the cave and take every option that sends you deeper in. At the bottom floor a random event will show you stairs leading up, theres a chance they will take you to Alluris's undercity, but also a chance to end up in the Dwarven mountain town, if you end up there you have to go to the main road and try again.
Once in the under city, just stay there until an event pops up where someone wants to sell you a mysterious map. Purchase it, then take the main road to the mountains. A new event with a waystone will randomly appear in the mountains that will take you to Baron Dreads fortress and pop the achievement.