Another of the frequently mentioned, but often overlooked races, the Yuan-Ti have great potential as villains - but much like the Drow, very little ability to do much more in a setting.
They're generic evil snakemen - they look like snakes, worship evil snake gods, etc. They're like serious versions of C.O.B.R.A.
So, I wanted to try my hand at a little reworking, for Delraith. We'll see how it goes.
The Yuan-Ti are a race of snake-like humanoids, hailing from the warmer Southern Climes - a land of hot, dense jungles and swamps. They stand roughly 6 feet in height, are bipedal, and have a distinctive scaled skin and serpentine features. Many have near-unique markings and colourations - no two are alike (and if they are, it's a sign of inbreeding or trickery). They have somewhat poor vision, and rely on reading ambient heat to "see" (although they are capable of true sight at close ranges). They are also adept at tracking via pheromones and scents, if need be. As such, they find it hard to identify outsiders at a glance - excluding massive differences (like the height difference between a Halfling and a Human, for example).
Originally, they claim to have came from 'another place' - some other dimension, perhaps, or another world. Their creation myths and the powers they worship are unlike many of the other Races of Delraith. They speak of this "other place", a paradisaical world of heat and water, where they are the only lifeform - almost like a "heaven". Some great cataclysm rocked this "Heaven", and the Yuan-Ti came to this world. How they managed it has been the Great Secret they keep, at the request of their God - a physical being, apparently, a huge Feathered Serpent who still resides on their world, waiting for the Yuan-Ti to release him from the cataclysm that imprisoned it.
They managed it via huge ships, powered by a mix of technology and arcane sciences. At the moment, these reside under their homes in the Southern Climes (each is roughly the size of a small town, and makes for a great high-level adventure/dungeon!). However, that was many generations ago - the Yuan-Ti have just about forgot how they managed it, but hold The Great Secret as a matter of course.
This Great Secret has informed a lot of Yuan-Ti culture - they are generally xenophobic, trying to keep themselves to themselves where possible. Outsiders are not tolerated within Yuan-Ti society - adventurers even less so. They have been known, however, to make deals with individuals of other races in exchange for information - sometimes mundane (like maps of the known world, or specific locales on the other side of the world), sometimes occult (like certain magical rituals, secrets of arcane might, etc.), and sometimes much deeper than most have the right to know (information about Dwarfholds, Dragons, and the nature of Elves). Such exchanges are done with great consideration, far from Yuan-Ti lands - normally in one of the neutral areas of the Climes, like the Elven settlement of Kust.
There are rumours that the Yuan-Ti have been experimenting with creating half-human hybrids, for espionage and to send out as scouts and spies into the other parts of the world. Most dismiss this as fanciful rumours - but there have been quite a few reported break-ins from large repositories of knowledge, like libraries and Wizard's towers...