Saturas marks the Mountain Fortress in the far south as the resting place of one of the Focus Stones. 
Lester is already there and joins you for the climb through a fortress full of Harpies, a book puzzle in the old library, and a long push to the stone itself. Bring a blunt weapon for the brute guarding the bridge.
It is a long trip south and a steady fight once you arrive, so pack accordingly.
- Healing potions.
- A good weapon with blunt damage.
- A bow or a backup weapon.
- A
Scavenger if you have one, since the fortress is far south.
Ride down to the southern tip of the map where the Mountain Fortress sits. Lester is waiting near the bridge. Talk to him before you go any further, because a large enemy blocks the way ahead.
Lester tells you to bring blunt damage to this one, so equip a blunt weapon and take down the enemy by the bridge. Loot whatever it drops. A short scene with Lester plays, and the quest proper begins.
Head in through the main entrance. From here on, most of what you meet are Harpies, which are not much trouble, and Lester pitches in on some of them. Cross the main open area, go right, and head up the stairs, cutting them down as you climb.
Carry on up to an open area and go right onto the balcony first. Clear the Harpies here, then look around for a chest and loot it. Lester should be waiting on the right, so speak to him before moving on.
Backtrack the way you came, loop around, and head up the next set of stairs. The room here holds the clue for the library puzzle, painted on a large picture. The order to remember is monster, then wolf, then mole rat.
In the library you pull books from three shelves in the order the picture showed.
- First, the shelf with the monster statue: pull its book.
- Next, turn around to the shelf with the wolf statue and pull that book.
- Last, go to the shelf with the mole rat statue and pull the final book.
With all three pulled in order, the fireplace swings open.
Step through the open fireplace into a hidden room and loot it, there are potions, tinctures, and odds and ends worth taking. Go up the stairs, turn right, right again, and climb the ladder.
At the top of the ladder, open the chest and take the loot, then turn around. There is a drop into another room below, so lower yourself in carefully. More Harpies attack inside, so clear them, then go out through the large hole in the wall, head down to the right, and jump down toward the final fight.
A wave of Harpies drops into this last section. Deal with all of them, and once the area is quiet, pick up the Focus Stone.
Jump back up the stone steps and look for Lester. He may be where you first spoke to him, down to the right, or back near the library, so check both. Talk to him one last time, then return to the New Camp, find Saturas, and hand over the Focus Stone to finish the quest.