During the Conquest, you ordered that Chorus thieves be spiked for stealing Disfavored supplies. Jacomus revealed that this was one of the locations, which doubled as a supply cache with built-in deterrence.
Points of interest
This small area is surrounded by Chorus corpses on spikes. Approach the western inlet to confront Hordesmen from the Chorus. Kill them. Loot the area to find jewels in the hidden sack in the center and the Imbued Rod of Strength on the impaled Blood Chanter.