Underground Octam (オクタム地下跡遺, Okutamu Chika Ato Noko, alt. "Octum") is located in the Sebucus Islands in caverns running underneath the city of Octam on the surface. Rem's prophet, Hari, is located here.
When word of the Mist's terrors reached Octam the citizens devised a plan to save themselves. A city was built within the caverns running underneath Octam with an elevator connecting the undergound with the citizens' former homes above ground. Underground Octam was a safe place for a brief period and the citizens lived somewhat normal lives - they had homes, religious service, and even a town hall with a mayor and guards. They all lived a dreary, yet peaceful existence and survived on mushroom patches that grew within the caverns.
Unfortunately for Octam's citizens it wasn't long before they discovered a new menace within the caverns - - gigantic earthquakes that would come at random, destroying the pillars holding the homes and swallowing them up into the earth below. Nobody knew when the earthquakes would come and everybody lived in constant fear of being swallowed into the darkness. The people of Octam were trapped, for they could not return to the surface where the Mist and the Seru ran rampant. Their prophet, Hari, said to be Rem reincarnated, was their only hope. However, he remained in a state of sleep for years at a time. Nobody could disturb Hari while he slept, for it was said that trying to wake him up would keep his spirit in the other world for eternity.
After being sucked through the vortex within the Stone Flower at the Gate of Shadows the Ra-Seru heroes travel through the long and winding caverns to reach Underground Octam. The purpose for their visit is to find the prophet Hari, who contacted the Ra-Seru heroes through a dream telling them that he will reveal the meaning of their dreams. It looks like a quaint town, but immediately after their arrival an earthquake erupts. They watch with horror as the earthquake destroys a pillar and the home on top of it, sucking it into the darkness below. The owner of the house, a woman named Debora, almost shared a fate with her house.
Vahn, Noa and Gala make their way to the city hall where the mayor was residing on the top floor. It was once strictly a place of business but it became a shelter for the many who had lost their homes over the years since the earthquakes started. After their chat with the mayor a guard comes running frantically up the levels of stairs, shouting that Hari has finally awoken, knocking Vahn down in his hurry. The mayor follows the guard to Hari's resting place and when Vahn, Noa, and Gala arrive they are told that Hari wishes to speak to them.
Hari is revealed to compose three bodies: Hari of the Past, Hari of the Present, and Hari of the Future. The heroes speak to Hari one-by-one and learn valuable information, including the cause of the earthquakes as well as warnings about future events that will take place. Once the Ra-Seru heroes listen to everything Hari has to say the bodies of Hari go back to sleep. After the Ra-Seru heroes leave the building another earthquake erupts and the pillar holding up the building Hari is in begins to shake. A guard runs out of the building and knocks down a woman, scrambling for his own life. Vahn attempts to run in and save the people inside, but Gala prevents him, realizing that they can do nothing. Hari and the nuns watching him become swallowed up by the earth.
As the heroes and citizens are brooding three spheres rise up out of the darkness below and Hari's voice speaks to the heroes. Hari reveals that he knew the earthquake was going to swallow him up and that they will meet again in an unlikely place.
The mayor of Octam had listened in on the conversation between the Ra-Seru heroes an Hari and he gives the guard orders to give them access to the entrance of the Fire Path located at the other side of Underground Octam, so that they may defeat the Mist's henchman causing the earthquakes. The heroes make their way down the Fire Path and come across a gigantic lake of magma. They manage to ride across the magma on a floating piece of rock and come across the henchman responsible for the earthquakes, a gigantic Minotaur/Centaur hybrid with a Sim-Seru on its head. The monster, named Xain, had been causing the earthquakes by charging at the base of the pillars and punching them with all his might. After a grueling battle the Ra-Seru heroes defeat Xain. It turns out that Xain was a Sim-Seru who was controlling a Centaur/Minotaur host body and in retaliation the Sim-Seru freezes all the lava underground before its death. Once the heroes arrive back up to town the mayor gives them permission to use the elevator leading to Octam on the surface.
Once the Mist is gone from the Sebucus Islands most of the people in Underground Octam return to the surface and set up homes there. However, some decide to remain underground after having lived there for so long.
- If the Ra-Seru heroes find Spring Salts within a treasure chest in Vidna they can give them to the mayor of Octam for a reward.