So, today I was completing act 2 and came back to east marsh with the dinamyte to acccess this area. After getting the Shrine, the Fang of the Bonehunter for the with in the prison, and the muddy note, I noticed in my way out some land, and as I got closer, I could hear sounds of foes wondering the area and what I believe is either aether ground or crystals.
How do I access that area?
Devil’s Crossing. Go outside to the east (first act), follow the eastern part of the map about a screen or three until you come to a house with a ramp on its right side. Follow the ramp and you’ll find a dynamite spot. Go inside, kill the boss and get the runestone.
Broken Hills. Go to the south-western part of the map (straight west of Steps of Torment, or straight south of Lutra). There’s a small path to on the northern side that opens up when you get there with the first runestone. Kill the boss, get the next runestone.
Asterkarn Road. Go north up on the ramparts. Once they end, the entrance is just below the stairs (you actually go east and cross under the ramparts once you enter the secret area). Again, kill boss and get runestone.
East Marsh. Mid-north part of the map, huge boulder. Opens up once you have all 3 runestones.