The wizard studied the tomes they stole from the temple of Vecna for a bit, and discovered that Vecna's followers had a straightforward plan for defeating Quasxthe and undoing the Inundation.
You see, Quasxthe's machine was iterated in each of the six Elemental Planes, and it worked by forcing the Lawful Plane of Earth out of its natural orbit, further away from the Material Plane, allowing the influence of the Hellish Plane of Water to increase, raising the level of the ocean. Merely destroying the machines wouldn't help, and would indeed make it impossible to deactivate them. To control each machine, you need the two keys, the two orbs. Which, as it happened, the PCs had on hand.
So the PCs trekked across the Deathly Plane of Shadow. On their trip, they crossed paths with a bunch of drow and big spiders, whom they defeated. The drow had two PCs captive, a scout and a knight, who joined the party. Then the rogue's player stopped coming.
One of the conceits of the Elemental Planes is that each is non-Euclidean in some way. In the Deathly Plane of Shadow, it's that, effectively, an infinite line is much easier to draw than a finite one. So when they came to the place that contained the Shadow iteration of Quasxthe's machine, they discovered that it was an infinitely-tall round tower. They broke in, and discovered a curved room containing a door, guarded by a drow. They defeated the drow and discovered a second room, exactly identical, plus an additional shadow creature. They defeated these, moved on to the next room, exactly identical, except the shadow creature was stronger. This would have gone on for an infinite chain of rooms, each one guarded by a group of creatures slightly more powerful than the previous one.
Eventually, they discovered that the key to ending the infinite chain was to utter the password, which obviously turned out to be "swordfish". This admitted them to a room that was, again, identical, except a.) it curved in the opposite direction, b.) in place of a far door it had the machine, and c.) it was guarded by a half-illithid drider. They made unfortunately short work of the boss, and then, because the wizard wasn't there that day, it took them a preposterously long time to figure out that the two hemispherical depressions on the machine corresponded to the two orbs.
Eventually they did, and figured out how to work the machine to de-iterate it from the Deathly Plane of Shadow. Doing so decanted everything in the vicinity to its home dimension, so the party was dumped back into the Material Plane. They would have landed in the sea, but the illithid cleric they had spoken to earlier helpfully redirected them back to his lair.
They didn't yet have Plane Shift, but the warforged knight helpfully remembered (because he made a knowledge check, so I reminded him) a bit of fluff I had provided: "A sophisticated ventilation system, possibly involving an actual portal to the Plane of Air, keeps all the inhabitants [of the engineer undersea city of Garzak] breathing." So they made a deal with an engineer submarine captain (the same one who had stolen the first orb from the sahuagin in the first place) to take them to Garzak.
So they made it to Garzak and convinced the Council of Elders to let them at the ventilation system. The scout, thinking to create flaming arrows, wanted to buy a hundred pints of lamp oil. I pointed out that this would encumber him substantially, so he bumped it down to a little over 50 pints, few enough that he could carry them without fuss.
There were some restrictions on the system, primarily that it only had the power to send solid matter through once a week or so. On the day that they actually went through, a third of the party wasn't there and another third was late. The swashbuckler/knight came back, though. So I said that people could only go through in ones or twos, with a few minutes in between each pair. So the knight and the swashbuckler/knight stepped through first, only to violently discover that the engineer outpost in the Chaotic Plane of Air had been taken over by gargoyles and vampires.
The knights managed to hold their own for awhile against an encounter designed for three times as many people. Then the wizard's player arrived, so the wizard stepped through and rescued them, though his Fireball killed one of the knights. Then I NPC'd the cleric and the scout coming through, and they used a scroll of Reincarnate to bring back the knight. He was a warforged, so I rolled on a handy construct chart, and he came back as a Dread Guard (retaining the (Living Construct) subtype).
The next session, they came across the big boss of the area, a vampire factotum. It didn't occur to any of them to use stakes, strongly presented holy symbols, garlic, or any of the myriad of other vampire weaknesses, so they only barely survived long enough for the cleric to Turn him. Stat-wise, vampires are way too powerful for their CR; their balance comes from their ludicrous list of weaksauce weaknesses.
In the kitchen, they found a PC bard and a couple of NPCs, kept on hand as snacks. Then they found the vampire's coffin. They hacked it in two and dumped it out two windows. Then the Turning wore off and the vampire came back.
And the vampire noticed that the scout had 50 pints of lamp oil on his person.
This being the Chaotic Plane of Air, it was a high-oxygen environment, so I ruled that the explosion of a pint of lamp oil would deal 1d6 damage.
The vampire had Scorching Ray prepared.
The vampire made his save (and had a Ring of Evasion, so he took no damage). The wizard was outside the range of the explosion. Everybody else died.
The wizard ducked into a Rope Trick, replenished his spells and hit points, and emerged again to challenge the vampire to a duel.
The wizard polymorphed into some obscure beast from one of the later Monster Manuals, and the vampire cast Keen Edge on his +1 Merciful Falchion, so it would be a critical threat on a 15-20. They went back and forth for a time, the wizard managed to chew away at the vampire's hit points slightly faster than he regenerated them back. But eventually the vampire managed to hit the wizard and bring him down to -1 hit points (well, to bring his non-lethal damage up to 1 more hit point than he had).
The vampire sucked out all the wizard's blood, then chucked him out the window, to sail through the Chaotic Plane of Air for 1d4 days until he rose as a vampire.
Then the vampire took the orbs and returned them to Quasxthe (and was lavishly rewarded for it). The campaign was over, unwinnable.