In short: A journey to a fort besieged by Goblins.
Difficulty level: Hard. I don't recommend taking it at the beginning of the game.
The quest requires you to get to the Shadow Fort - a fortress in the south of the land. You can get there by taking the passage through the Ancient Quarry. The entrance to the quarry lies southwest from Gran Soren (as seen on the map above).
Right before the Quarry there is a merchant with two quests - you can do them at once, that way you'll clear out the way through the caves. You'll be able to run through them without loosing stamina (just as inside a city).
Once on the other side, follow the road to the fort. Don't get close to the lake - a dangerous Drake lives nearby.
Enter the fort.
Talk to the local commander in front of the main gate. You need to get inside - use the underground passage not far from the troops.
You'll find yourself in the building controlled by Goblins. Eliminate them and use the stairs.
The stairs will lead you to the main courtyard. It's a large location with a multitude of enemies, including two Cyclopses. Ignore them all for now. On the right, there is a huge gate (you'll open it later). Climb the ladder on opposite the building.
Take care of the guards.
The next thing to do is destroying the two ballistas on the other side of the courtyard. Enemies will use it to attack you. Use the ballista on your side. Switch to incendiary missiles and destroy the enemy nests; aim at them separately and keep firing until you see an explosion (your missiles will "bump into" the enemies', so you need to fire continually until they reach the target).
Once you've cleared the way, climb down and go the ruins on the left - Station Room (halfway through the courtyard).
The main entrance to the ruins (the gate) will be blocked on the other side. To get inside, use the ladder on one of the side walls.
In the Station Room, go through the prison door and get rid of the Goblins.
On the other side of the prison, take the lever from the chest.
With the lever, return to the main gate to open it. The exit from the Station Room is probably blocked by Cyclopses - jump between their legs. The mechanisms to attach the lever to is located on the roof of the building on the right, by the walls. Open the gate.
Soldiers will appear on the courtyard - but their help is rather small. You need to kill two Cyclopses and Goblins. It's best to use the siege machine you've been shooting from before.
When the Cyclopses are down, enemies will retreat to the next part of the structure. Follow them. The road to the Goblin leader is on the right (inside the building).
Climb up the winding stairs. You'll meet another Cyclops there - you can fight it or ignore it. Continue through the corridors and go back down through the second tower.
Finally, use the double wooden door and fight the Goblin leader (he's not particularly difficult). When it's over, talk to Ser Robert and return to Gran Soren (to get the reward from Ser Maximilian).