During the Subterranean Life quest, you encounter a mini-quest in The Witcher 2, Hey, Works on In the Mine! In this quest, you will find a special herb, Dwarven Immortelle, that grows in underground areas. Philippa will need this to make a cure to save Saskia.
To complete this quest, you must bomb the nest points to shut the access of Rotfiends and save the Mine.
As the quest starts, you must reach the Dwarf friends in the Inn. Ask them about Saskia and the herb; they explain that you can find it in the mines. Unfortunately, the mines are closed off due to the presence of some monsters.
The basic task of this quest is to bomb the three sites where the nests of Rotfields are to eliminate them and block their access. For bombs, you’ll need Grapeshot bombs, which can be bought from Earso in The Witcher 2. Here are the locations of the three nest points.
After the first set of Rotfiends, take the right turn towards the door and go through it. At the end of this area, place the Grapeshot bomb. Also, loot the body as it will reveal an Upper Shaft Key, a Map of mines, and Balin’s Journal page #1.
Take the left path now from the intersection and open the door here using the Upper Shaft Key. Now, go around the longer path, defeating Rotfiends and looting the area until you reach a split. Take the left side; at the left corner of this area, you’ll find a body with a Lower Shaft key and the second page of Balim’s Journal.
Turn right, and you will find the second nest spot for the Grapeshot bomb in The Witcher 2 Hey, Works on In the Mine! Before the third nest, you first need to get the Storehouse Key. For this, go down towards the split path.
Choose the left one and keep going until you reach the end. In the last room, loot the body and get the storehouse key. Be careful, as this place has a lot of traps in The Witcher 2: Assassins of Kings.
Now, backtrack and stop at the split. Take the other path this time (right) and go down until you reach the end. After defeating the Bullvore, go towards the end of the area and plant the grapeshot at the last nest location.
After the blast, the quest will be complete, and you can visit the Cecil to receive 50 Orens and the Formula: Hardened Leather Boots.