What dungeon?
Ah that, in the path to the orc camp there is a hidden path you can unlock. You see those small balls hanging on the trees? well click on them in a specific order and it will open a path which is the location of one of the sigils for the ancient door scene and also were the entrance to the mansion is. Just try every possible combination until you get it, the balls stay glowing if it is the right one but all of them turn off if the one you pressed is the wrong one.