Here follows the speculation I made about the existance of the Kopru Ruins below Cauldron. Purposefully, I chose to not work out a timeline.
I thought to a sad story to bind to that particular place. (Obviously my players didn't even suspect for a single moment to be in kopru ruins...).
Let's start with consideration: Kopru should live in salt water, while this particular site in a sweet water place, an exception to the rule... So, this discrepancy led me to build the story.
Perhaps, here begins the tale, some of the kopru realized earlier than others of their kind that their race was doomed to an unstoppable decline. The matriarchs strongly opposed any speculation brought before them, thus the race quickly started its fall.
Some of the koprus worked in the shadow and agreed to try for what they considered their last chance of survival. They knew that only a bunch of them would have been successful in the task.
With that knowledge in mind, they eventually started a long land journey from the sea to the only place they found worth of their effort: a lake, that despite being a sweet water one, had the biological condition to afford the koprus able to reach it, a decent chance of survival.
The journey was a massacre. Koprus started losing members day after day. Their slowness on firm ground made the perfect prey for the predators living in that savage area. Helpless, one after the other, they fell to the jaws of the wilderness. At the end, as it was foretold, only a few of them got the lava tubes and the subterranean lake (at that time there was no suspended cage and the waters were higher enough to let them dive safely in what is today the crevice).
However, some of them never accustomed themselves to the sweet water and their biology started changing. Their offsprings were born more with amphibian traits and they were not so helpless outside water. This mutation enabled them to even realize buildings on firm ground.
While the rest of the koprus were dying, they prospered, and created the small dwelling that later will become the base of the Ebon Triad.
But nothing is forever... and eventually their time came. Once they finished what is now the Kopru Ruins under Cauldron, they lost their motivation in surviving, they felt safe. Soon after, perhaps starting from the following generation, they started the degenerative process already experienced by the rest of their race.
They become ever more decrepit and their chaotic and evil nature led the strongest to kill the weakest in a desperate effort to put a stop to the degenerative process. This was not enough, as many of the subsequent generation were stillborn. Their number grew ever more low and when the last of them remained, it was the one who showed the necessary combination of willpower, luck and motivation needed to survive. That lone exemplar still lives today. For some mysterious reason, he does not age anymore (he is aware of this and tends to hide) and lives comfortably in sweet water (unknown to it also in salt water) as well as on firm ground.
Should another kopru with that characteristic exist somewhere in the world and should they encounter and mate, the race of the koprus could rise again...
Of course, there are a lot of biological mistakes (try to put a salt water fish in sweet water...) and perhaps the timeline could not be worked properly as the Cauldron region seems to be subjected to earthquakes that could have changed the subterranean tunnels in time. However, I hope someone could find this helpful...
Keep it coming man! This is very helpful.