For The King II

During the 2nd campaign I came across the Mausoleum in the foothills, it was near the southwest area of the map for me, this game is roguelike so unsure if the Mausoleum will be in the same spot every time. The achievement popped when Gaveborne was defeated(mid battle)

Yellow tiles mean your party hasn't search the area and may still have goodies to unfold.

I also came across the mausoleum in the 3rd chapter. It was a tade stronger than the chapter 2 battle but it appears you can find the mausoleum in multiple chapters if you're searching the yellow tiles.