Within that realm, pain and suffering have a terrible purpose. Those individuals who perish there have their souls as well as their very essence captured and placed within the Soul Tombs. It was a large palace that was accessible through the Portal with a large number of guards present within it and it is indeed built like a prison. An Orochi Hellbeast was also present which was risen from its chamber when intruders entered the tombs.
They were said to have been the source of Shang Tsung's power which was why Raiden asked Liu Kang and Kung Lao to destroy them. When inside the Soul Tombs, they both discovered Shang Tsung was there regaining his youth and recharging his powers before they destroyed the strange machine that restored him and is part of the tombs, guarded by the Shadow Priests after he disappeared.
The Soul Tombs are where both Liu Kang and Kung Lao also go up against Baraka who served as the boss of this area.