Last session in my 4e game the players were exploring the dungeons underneath the royal palace of Siss-Anor for the first time. Magical wards prevented teleporting into or out of the area underneath the palace, so for the first time in a long time the party didn't have the safety net of being able to open a portal back to their orbiting spacecraft with their portal coins. I am going to post some maps of areas of the dungeons, because by the end of last session Beautiful Bob had wrested control of two cambion (half-devil) slaves from a royal sorceror. These two cambions are now totally loyal to Beautiful Bob, and they are intimately familiar with the areas of the dungeon occupied by the devil-pact Order of the Horned Bull as well as the network of passages and caverns that link from the order's temple to the ancient ritual caverns in the lowest level of the dungeons. I am going to post some maps soon, but first some back story.
For the last few months of game time, the city of Siss-Anor had been oppressed by the snake men who suddenly arrived in their flying golden city/meteorite over the palace and activated sleeper cells of snake men loyalists within the royal family and royal inquisitors. The royal bloodline of House Al Treez was revealed to be of snake man scientist creation - genetically modified to be susceptible to snake man telepathic command. Squadrons of large humanoid lizardmen with green shadows instead of heads abducted most of the ranking leadership of both the other royal houses and the named merchant houses. Unprecedented numbers of blood sacrifices were demanded of all the houses, and several resulting uprisings were put down with brutal efficiency by the legions of the snakemen. Then just as suddenly as they arrived, the snake man city flew away upriver into the jungle and has not returned.
Without the backing of the snake men, the collaborators within the old royal government soon were besieged in the palace. When the palace fell the old loyalists/snake man collaborators retreated into the dungeons below the palace. The royal sorcerers, while not of snake man blood and in fact chafing under the new order, had not openly resisted the snake men and indeed had helped put down the uprisings. A secret society within the royal sorcerers known as the Order of the Horned Bull had formed a pact with the lords of hell many generations ago, and they called upon the minions of hell to protect them as they retreated into the dungeons. The upper level of dungeons, mostly consisting of barracks for the royal guard and extensive prison cell blocks, are now flooded with devils - at the moment keeping the forces of the uprising houses from capturing the collaborators.
The party has a man on the inside, a member of the royal family and a royal inquisitor himself, Kaaz al Treez. Many sessions ago Kaaz allied himself with the party because he had been investigating secret plots of the snake men in the royal house of Siss Anor for quite some time. He had been investigating a secret order within the inquisitors known as the Order of the Serpent. This secret order passed down the knowledge of the snake men for thousands of years, conducting rituals in hidden caverns far below the surface where a strange race of snake-mindflayers (snake man scientist creations) live in a network of tunnels honeycombing a sentient mycelial mass. Even after the the snake man citadel left, Kaaz observed snake man scientist activity in the lowest levels of the dungeon where the snake-mindflayers live. Literally taking the form of a fly on the wall, Kaaz observed a snake man scientist entering a secret doorway in the Cavern of the Blood Sacrifice. Following the scientist in his insect form, Kaaz discovered a snake man scientist installation hidden underneath the caverns. A large circular chamber was filled with the golden machinery inset with gem buttons and festooned with levers that is characteristic of the snake man scientists. In the center, some kind of energy vortex or singularity danced suspended in mid-air. Kaaz observed the snake man scientists walk directly into this energy vortex and disappear. When Kaaz followed after a discrete moment, he found himself on a clear platform far above the floor of an immense chamber. Hairless blue skinned humans and snake man scientists of unusual coloration busily crisscrossed the floor of the chamber to and from the eight archways ringing the chamber. Something in the atmosphere felt wrong, and he had difficulty maintaining his insect form. He was unable to communicate with the party from in this chamber through the magical wasp ear rings that he and Beautiful Bob wear for this purpose. When he attempted to fly down into the chamber he lost his insect form and crashed into a far corner of the immense chamber. He had difficulty using any magic at all and was unable to assume any of his tattoo forms. Eventually he was able to figure out how to activate an elevator lift to take him back to the platform, where he stepped back through the energy vortex. He contacted the party after this and told them the whole story. He explained to the party how they could access the lowest levels of the dungeon through a long flooded passage connecting to the old sewers, and that is how and why the party came to be entering the Cavern of the Blood Sacrifice underneath the palace of Siss Anor last session.