from the wiki:
Exit Vergen and walk past the burned village, head to your left through the stone archway. You will encounter a small shrine, with three paths open. The right path (to the right of the shrine) leads up to a dead end. The center path leads you to Elthon's hut and the old quarry. The left most path will lead you to to an entrance to the Tunnel of the Founders. Take the left most path.
The entire region here is very dark, so you must use Cat potions or Flares in order to progress through the mine. Common enemies include nekkers and rotfiends, along with two bullvores. There will be a clearing in the mine where you fight a bullvore, past that room you will find Zoltan and Saskia, who will freely give you the Commander's sword."