A hidden network of underground caves and grottos beneath the Forgotten Forest, accessed from the River Trail.

You dive into the river and swim toward the underwater caves. Pulling yourself through a winding maze of limestone, the underwater tunnel eventually leads you to a rocky shore inside an enormous cavern. Many spirits have gathered here. Their blue and white spectral forms radiate with a soft light that bounces off the rocks and the surface of the water. Otherwise, the caverns are pitch-black. - Ghost-Eaters of the Forgotten Forest (RPA-ADV7)

A diary entry by Lady Dorothea details how the spirits of the caverns found themselves here;

It was the height of arrogance for my father to allow the caverns beneath the Forgotten Forest to be mined for precious gems. It was only a matter of time before tragedy struck, and no words of comfort will help the families of the fallen. I hope, at least, that their deaths were quick.

We have contained the most dangerous spirits in stone warding circles, but I fear it will not be enough to prevent them or their descendants from seeking their revenge. - Lady Dorothea Duntum, Ghost-Eaters of the Forgotten Forest (RPA-ADV7)

Using the Illuminating Stone further reinforces the events that took place;

Briefly, you see the image of miners chipping gems off the walls. The cavern shakes and collapses around them, burying them in the rubble.

When the vision fades, you search the cavern, discovering skeletal remains. Some of the corpses carry mining tools and other equipment. - Ghost-Eaters of the Forgotten Forest (RPA-ADV7)

After jumping into the river to save the pig with a black, swan-shaped spot, you eventually surface in the caverns below:

• If you let Creet feast on its blood in Sabek:
  • You defend yourself and slay the charging pig.
  • You are bit and become infected with vampirism.
• If you rummaged around the mud the pig was sniffing at the River Trail before coming to its rescue:
  • A spirit girl explains that the pig is upset you did not jump in after it, and is looking for her family.
• If you jumped immediately into the water to save the pig:
  • A spirit girl explains that the pig is looking for her family;

“She’s not a pig, she’s a girl like me. She misses her family. Especially her brother, Jeremy. Oh, she’s so much fun. I’d rather she stay here, but she wants to find her family.” - Ghost, Ghost-Eaters of the Forgotten Forest (RPA-ADV7)