Hall of Serpents

The walls of this pillared hall bear reliefs of a giant serpent and its yuan-ti worshipers doing terrible things.

The wall reliefs show the legend of Dendar the Night Serpent and the rise of Ras Nsi. Characters who examine them see the following tableaus:
A giant serpent coils in the underworld, devouring the dreams of sleeping creatures. Yuan-ti terrorize Chultans, feeding their nightmares to the great serpent. A yuan-ti malison (Ras Nsi) wielding a flaming sword and wearing a gold crown set with a black opal leads the snake people to a great door to the underworld. The door to the underworld opens, releasing the serpent. It devours the sun and plunges the world into terror and darkness. A character who searches the reliefs and succeeds on a DC 15 Wisdom (Perception) check discovers that the black opal in Ras Nsi’s crown is a stone button. If the check succeeds by 5 or more, the character also sees tiny holes hidden among the scales of the Night Serpent carvings.
If a character presses the button in Ras Nsi’s crown, the carvings of the Night Serpent expel a soporific gas that fills the hallway and lingers for 1 minute. Any creature that moves into the gas or starts its turn inside it must make a DC 13 Constitution saving throw. The gas has no effect on creatures that cannot be poisoned, and creatures that fail the save fall unconscious for 1 hour. Using cloth or wax, characters can plug the hundreds of holes and prevent the gas from flowing out.
Navigation
Heading North leads to the Hall of War, there is a door on the eastern wall that leads to the Armoury, and to the South East is a staircase leading 30ft down into a hallway with multiple exits. These exits are the Storeroom on the first right, the Venom Distillery on the second right, the Snake Pit straight ahead, and The Fane on the left.