In terms of game statistics, the Tiamati are nearly identical to the Dragonborn presented in the PHB. Make the following changes:
Languages: Thalind Pidgin, Drow
Skill Bonuses: +2 Dungeoneering, +2 Intimidate
Also, disregard the fluff text for the Dragonborn, as the history of the Tiamati is quite different indeed.
The Tiamati are an enigma. Brought out of the earth by the Drow as slaves, they can now be found throughout the known world. The Tiamati themselves know nothing of their origins--were they found and subjugated by the Drow, a result of some alchemical experiment, or born of some unholy union? Being that the Tiamati are slaves or children of slaves, most are poorly educated and taught to lead a brutal lifestyle. However, those who are free value their freedom and the opportunity to advance themselves.
Most Tiamati adventurers will be freemen. This means one of several things. They escaped from their masters, earned or bought their freedom, or they are the child of a Tiamati who did one of the above. Most Drow will not permit their slaves to go free, so most Tiamati who earned their freedom did so after being sold to a non-Drow master. In Abar and the Northlands, slaves of Humans are afforded a great deal of social mobility, and many Tiamati have earned their freedom in these places. In Porre, slavery is illegal. Whenever a Drow master is punished for his crime, he must free his all of his slaves. Many Tiamati so freed find themselves in such financial hardship, however, that they return to their former masters.
Freed Tiamati have differing perspectives on the Drow. Many despise the dark elves for enslaving their people, and either shun their company or seek to bring harm to the Drow. On the other hand, many Tiamati find themselves the objects of distrust by the "surface races," and find that the Drow share a common language and are willing to pay modest wages for a strong laborer.
The Tiamati tend to follow the religion of their masters. This creates an interesting situation in some Drow households, where the nominal religion is Aurism, but the family secretly follows the Infernal traditions of their ancestors. Where this is the case, the Tiamati slaves usually practice Aurism without knowing their masters' true spiritual persuasions.
Medrash knocked twice on the side of the parked wagon. As he waited for a response, he looked out over the dark waters of the Placid Sea. A strong wind blew off the water and made Medrash shiver a little. Though other inhabitants of Seastar complained about the heat, Medrash didn't mind it at all. In fact, he and other Tiamati hated to go out at night, when the sun stopped beating down on their scales and the temperature dropped.
From inside the wagon, a complicated return knock signaled that Medrash was to climb inside. He hoisted his enormous mass up into the back and took a seat in the dark. He smelled the familiar stench of dried blood, and felt around with his hand. His fingers came into contact with the ruddy bearded face of a Dwarf. Another disposal job...
The wagon came to a stop near the rushing water of the river. He heard the driver hop down nimbly from the front, and saw him peak up into the back. He was a dark skinned Elf with white hair and a triangular scar on his right cheek.
"Come on, Lizard," the Drow said, "we gotta get these bodies into the water. Don't drag your feet."
Medrash despised Drow, especially this one. His name was Xulgos, and he was the boss's little brother. Among the Drow, cruelty was common, even between family, and it was no different with the Fillith clan. Xulgos had been branded by his sister for some failure or another, or perhaps simply for her amusement. Xulgos was no victim, however, whatever ill treatment he took, he simply passed on to lesser members of the guild--thugs like Medrash.
Medrash hated all Drow. But it hadn't always been that way.
Medrash had been the slave of Ursma, an Abarian diplomat, since he was a child. He had worked diligently for Ursma in many capacities--bearer, guard, stable-hand--anything Ursma asked, Medrash had done. After many years of service, Medrash had saved enough coins to finally buy his freedom from the man. He fondly remembered the day when Ursma, with tears in his eyes, broke the iron bracelet symbolizing his slave's servitude, and thanked him for years of loyalty. Medrash wished Ursma peace and prosperity for him and his family, and left the estate.
For several weeks, Medrash savored the freedom to walk where he wished and do as he pleased. He eagerly presented himself to Abarian merchants around Seastar, hoping to find a job with the help of Ursma's letter of introduction. But they all turned Medrash away. Soon he was running low on coins and hope.
When Myrymma, a Drow Sorceress, approached him and offered him fair wages to serve as a guard in a warehouse, Medrash was eager to accept the offer. He had heard from other Tiamati that the Drow were despicable and cruel, but Medrash had never worked for one himself, and figured the tales were an exaggeration. Besides, he would be an employee, not a slave.
Myrymma provided Medrash with good equipment, hearty meals, and steady work, but the Tiamati began to become suspicious of the Drow. First, Myrymma insisted that he pay her for the equipment, food, and lodging. She said she would keep his account and pay him when he broke even. Worse still, Medrash was soon asked to do more than guard a warehouse. His considerable strength was put to use breaking the bones of Myrymma's debtors and fighting clandestine battles against cloaked humans in dark back alleys.
Now, Medrash knew he was again a slave. He knew this time, he would never escape. The grasp of the Underbelly was too strong and stretched too far. Nowhere in Thalind was safe.
Medrash toiled alone to carry the bodies to the steep bank of the river, tie weights to their legs, and throw them in. The entire time, Xulgos cursed at him, spat on him, and muttered to himself.
"You have it easy, acid breath, no responsibilities. Just do what you're told, and the dark elves will figure everything out. We put a roof over your head, feed you, all for mindless work. When assassins come from the Merchants' Guild, they aren't gunning for you, they're after dark meat."
Medrash kicked the last body into the water with a splash.
Xulgos continued, "Not once have I heard one of you monsters say thank you, either. You beasts eat us out of house and home, and you don't care one bit. Stupid filthy ingrates."
Medrash closed his eyes and flared his nostrils, then breathed in calmly. He turned around and looked Xulgos in the eyes, before roaring forth a spray of corrosive acid all over the Drow.
Xulgos fell to the ground, scratching at his skin, his now blind eyes wide open as he breathed in to scream.
The scream never came, as Medrash's scaly foot had crushed the Drow's head before he could let the air out. This was the first kill in Medrash's war against Myrymma. He would rather die free, than be a slave again.