Exploring the Frontier Caverns and Fighting Off Assassins

Exit Gran Soren via the southern gate and talk to Ser Rickart on the bridge outside. He will give you an 'Unsigned Letter' indicating someone is looking for you.

Rest at an inn until nightfall to ensure assassins appear.

Travel to the Deos Hills. After crossing the bridge from Estan Plains, head north-east along the road until you reach a split near some rocks. Here you will encounter the first group of assassins: Phantom, Ghost, Soul, and Spirits, along with Skeletons.

Make your way to the Ancient Quarry North Entrance. You will find another group of assassins: Raven, Hawk, and Condor. Defeat them and proceed through the Ancient Quarry.

Locate a large Goblin hole near the walls of the Shadow Fort. This hole is south-west of the unmarked structure along the northern tower, east of the front gates, and in front of wooden scaffolding.

Enter the hole and follow the tunnel north-west to reach the Frontier Caverns. You will unlock the trophy/achievement 'Into the Frontier Caverns'.

Head north, then east to a chamber filled with Goblinoids (normal and Hob). Fight your way east to a small side chamber.

Loot the ledges to the south and east of this side chamber. The eastern shelf contains two Coin Pouches, two Large Coin Pouches, and a Giant Coin Pouch, totaling 19,000 gold. The southern ledge is filled with pots, explosives, and Foreign Knives.

Peruse the northern tunnel you previously neglected. Follow the long, narrow path filled with Goblinoids heading south until you reach stairs.

Descend the stairs into the 'Proving Grounds' arena. This area contains several Goblins and an armored Cyclops, the target of the quest 'Put the Eye Out'.

Kill the Goblins, then scale the Cyclops to make it remove its helmet. Shoot the Cyclops in the face. Once defeated, you will receive the 'Silver Idol' if you started 'Put the Eye Out'.

Search the perimeter of the Proving Grounds for two chests located east and west of the large northern gate. These chests contain high-quality loot for this stage of the game.

Return to Gran Soren.