To trigger the dungeon mission, you need to talk to the lost knight four times on the map, at least one day apart each time, and then he will give you a treasure map. The first time the lost knight was in Dugan, Adlen, Tebanda or Fedna. You should look carefully for these four villages, maybe in the Woods. If you meet the lost knight for the first time, you can ask him if he can falcon eyes, and you can easily find the lost knight. Underground city entrance. Take the treasure map to the lake on the edge of Gilkala, stay by the lake and find the entrance.
Enter the bottom of the lake and follow the stones. First, walk along the stone on the left to the end. There is a treasure chest.
Go back to the fork in the road and follow the stone on the right, and you will arrive at the underground city.
Next, you explore slowly.