You breathe a gout of flame at the top of the central lantern pillar, which does appear to have some residual oil in it, based off the bright yellow flame that results. With the central lamp lit, you can now see clearly around the rest of the chamber; the walls are covered in bas-relief carvings depicting what appears to a history of the Ophidians, mixed in with a fair amount of myth and religious allegory. According to the scripture on the walls the Ophidians were descended from the Naga, the chosen of Fortuona, the goddess of Luck, Passion and Vengeance, and carried many of her blessings in the process. They forged a powerful empire, subjugating the nearby mortal races, raising up various Princes and also forming a healthy cult to Fortuona's sister deity Ush-Idar, the (sexless) god of Death, Destruction and Despair, from whom the secrets of undeath were apparently learned. They apparently mostly used the undead as ceremonial tomb guardians for most of their history, preferring to employ living minions as servants and warriors in battle.
Unfortunately, the whole empire went tits up when they forsook Fortuona for her brother and enemy Sidim, Father of Chains, the Brother-Binder, the Architect, the God of Civilisation, Slavery and Law. Having been tempted over for many generations they underwent a massive societal change, enslaving most of their former client races and building up new temples to Sidim. One thing led to another, the primary temple to Fortuona got desecrated, and so the Goddess shattered the core Ophidian empire in a massive earthquake and sunk their grand new city into an enormous swamp. Probably the swamp over your head, all things considered.
The later sections of the dome are a bit depressing. The Ophidians were abandoned by Fortuona for their treachery, by Sidim because all of his temples went down in the earthquake, and by their client races/former slaves who turned on them and butchered most of the Ophidian race, forcing the survivors down into the ruins of their city, of which most of the surface was destroyed but their ancient necropolis survived. The last carvings, which lack the beauty of the earlier reliefs and appear to have been placed by an inexpert hand, tell of how the survivors sheltered in the cult of Ush-Idar, the only deity who did not desert them (and whom they did not desert) and lived amongst the dead, raiding the surface world for food and supplies and building up an army of corpses to take back their lost empire.
You haven't seen any empires in the swamp above, so you have your doubts about how successful that campaign was. Still, these tombs do seem to be part of an expansive necropolis underneath the swamp, the result of centuries of tombs being dug and interconnected, plus some surviving sections of submerged city. Probably crawling with undead, if this one is any indication.
On a practical note, it seems that most of the tombs here have minimal interior decoration that isn't beautiful carvings - they would be valuable, but only as part of your lair. However, it seems that burial of valuable grave goods is standard. This chamber is part of Prince Narsis' crypts, as is the long corridor you went through before and the main pyramid chamber; in this chamber are buried favoured servants of the Prince, the long chamber has warriors to accompany him into the afterlife and protect his body, and the pyramid chamber has both the Prince himself and his elite guard.
A) Favoured servants, eh? Start opening caskets in this chamber and get to the looting, they can't be that strong.
B) Rank and file warriors? Taking those on would be a terribly intriguing challenge.
C) Let's not bother with the side shows. The Prince himself offers the greatest challenge and reward.
D) Let's do something else.