The party successfully solved this puzzle, unearthing the "Blade of Omu" (vorpal sword) from within and recovering the last experience of the baelnorn during her spell duel with Acererak.

The obelisk was surrounded by a permanent incendiary cloud which emulated planar conditions of Carceri (i.e. no escape). It had 3 rotating levels and Old Omuan inscription at its base: “Bound by devotion to the Nine, and enduring service to our Queen, all citizens, above and below, fear the blade of Omu at their neck.”

The images, from left to right, top to bottom, represent races that were subjects of the Omuan empire: Aarakocra, Batiri Goblin, Tabaxi / Grung, Human, Dwarf / Lizardfolk, Minotaur, Kobold.

Their arm positions corresponded to racial attitudes toward worship – upraised hands indicated worshippers of gods (Aarakocra, Dwarves, Lizardfolk), downturned hands indicated worshippers of primal entities that were not divine (Batiri Goblins, Grung, Minotaurs), and mixed hands indicated secular races or races which vary greatly by individual (Tabaxi, Humans, Kobolds).

The coloration corresponded to activity cycles of the race – pale face indicating diurnal races most active with the sun (Aarakocra, Humans, and Minotaurs), dark face indicating nocturnal races most active at night (Batiri Goblins, Dwarves, and Kobolds), and mixed faces indicating crepuscular races or races that don't conform to normal activity cycles (Tabaxi, Grung, Lizardfolk).

Each line of the inscription was a clue about how to rotate the obelisk in order to open it:

"Bound by devotion to the Nine" hinted at the Omuans skewed belief that the Trickster Gods were actually gods, and was solved by aligning all the upturned worshipful hands along the same face of the obelisk.

"enduring service to our Queen" hinted at maintaining vigilance throughout the day and night, requiring aligning the races according to activity cycle (e.g. with all nocturnal races aligned on one face of the obelisk). 

"all citizens, above and below, fear the blade of Omu at their neck" hinted at unification through fear, and was solved by returning the obelisk to its starting position or rotating it 360-degrees (which worked out to be the same thing).

A wrong move (or attacking the obelisk) emitted a 20-foot radius of blinding light; DC 20 Con save or 4d10 radiant damage, and if reduced to 0 hit points, their eyes are burned out.