Okay, here's a bit more:
Yuan-Ti: These demon worshipping snake people, like the aboleths and illithids, benefited from the Catastrophe more than they lost from it. They did lose, almost everyone lost in the Catastrophe in some way, but they also accomplished much of their agenda. They worshipped demons and serpent gods, both of which died during the Catastrophe, but their society did not fall. They still have slaves, a whole race of people under their complete control (broodguards) and the warlocks did not lose their power. Still, they were disoriented from the Catastrophe. Mershaulk and Sseth seemed to have died, and the only patron of the Yuan-Ti that was replying was Dendar the Night Serpent, who their whole societies started revolving around.
Over the centuries, their culture became more and more engrained in nightmares, sleep, and started being seen as the bogeymen of Toril. Becoming a nocturnal race of stealthy snake-primordial serving monsters, they sent spies across the world as Dendar commanded them. The race being changed by Dendar the Night Serpent, gaining the power to curse persistent nightmares upon unsuspecting creatures. Yuan-Ti Purebloods are effectively wiped out, and are all replaced by this
race, or this
stat block.
After centuries of scheming and serving their awful lady, Dendar the Night Serpent, the Yuan-Ti were finally able to free her. With her strength greatly increased by her servants' terrible devotion, she was able to communicate with her followers and reveal her location. Hidden behind an iron door leading to the Underworld underneath a mountain in Chult, she guided her servants to her foul location, who freed her from her prison 666 years after the Catastrophe. The god Ubtao, who was tasked with guarding this gate had abandoned the people of Chult, taking the city of Mezro with him. Fortunately for the world, he did not die during the Catastrophe, as he was not a resident of the Outer Planes.
With Dendar finally freed, she unleashed her horrific terror upon the world, at first devouring the Yuan-Ti who were present at the moment of her freedom. The Night Serpent flew south, devouring everything in her path, from trees, to stone, to earth and all the creatures that were in her path. Arriving at the Snout of Omgar, she devoured the island, growing in size, and destroying the homeland of the Tortles. Then, heading north, Dendar spent five days assailing the continent of Chult, destroying much of the jungle, consuming many undead residents of the jungle, and causing a cacophony of screams in her wake.
On the fifth day of Dendar's freedom, Ubtao sensed her release. He had been awarded the continent of Chult for his help in the war against the primordials at the beginning of the world, and was tasked with guarding the gate to Dendar's prison. Feeling a sense of duty to the world, Ubtao returned with the holy city of Mezro, and he, with the help of his barae, were able to re-imprison Dendar beneath the mountain in which she resided, and promised to stay watching that gate until the end of time to make sure Dendar never was released again.
After Dendar's re-imprisonment, with the population of Yuan-Ti significantly reduced by their "goddess's" devouring of her releasers, they hid in the shadows of the world. Still plotting for the day when they would re-release Dendar, they glide silently through the night, spreading horrible dreams through the minds of unconscious creatures, slowly, but surely increasing her power once more.
Tortles: 666 years after the Catastrophe, Dendar the Night Serpent was released by the Yuan-Ti inhabitants of Tor-eal, who immediately devoured the Yuan-Ti, many Chultans, and the island known as the Snout of Omgar, which was the homeland of the Tortles. Many Tortles perished during this event, and they suffered greatly in the following days as Chult was ravaged by the serpent primordial for multiple days, which was the location that had the second biggest population of Tortles, which caused even more deaths among their species.
At this point, the Tortles were restricted in places they could go. They didn't want to dwell on the mainland near other races, as they were suspicious of the more monstrous races, as is understandable if your home was just destroyed by the actions of a species of snake people. The oceans were controlled by the Aboleths and Krakens, and were not safe. They decided to start dwelling in lakes, only surfacing when they needed air or food. They used rivers as "roads" to transport them as they need for travel. Over the course of the centuries, they gained the ability to breath water, swim better, and developed underwater cities in lakes, and literally transformed rivers into roads that they could use to transport goods with carts pulled by large fish.
Here are their new racial statistics.
Tieflings: Due to their infernal nature, tieflings faced major discrimination shortly after the Catastrophe. Many of them were wrongfully blamed for causing or allowing the events that took place to take place, but the fault was not on them. Even some types of celestials either placed the blame on these tieflings, or didn't speak out when tieflings were discriminated. The main type of people who did not reject the tieflings were orcs, half-orcs, dwarves, and coatls. Throughout many centuries of getting closer to these people, tieflings eventually bred with coatls, causing their physical appearance and statistics to change.
Now, their became only two types of tieflings, one known as the Holyscales, and the others known as the Brightwings. Their horns and tails no longer were fiendish, but instead became scaled. Tieflings stopped being fiendish colors, instead becoming white, gold, yellow, green, orange, turquoise, or a mix of two or more colors. Their legs were also scaled, and the Brightwings had rainbow-colored feathery wings, like a coatl's. The Holyscales often have glimmering scales, often of the lighter colors, and they have the ability to sense evil and good creatures, detect thoughts, and call holy fire.
They now have a culture of right-doers, priests, and justice-bringers. Often becoming judges, clerics, and servants to the celestials, these tieflings are found scattered across the world, even in gnoll cities, or orc camps. They now also try to fight evil whenever it surfaces, also often capable of rallying people to their aid in these holy tasks.
Edit:
Here's the tiefling race
