Yuan-ti were originally humans who transformed themselves into serpent folk through ancient rituals. Most yuan-ti were corrupted into monsters by those rites, but some yuan-ti instead became a new people who mix characteristics of humans and snakes.
Blessed with resistance to magical and poisonous effects by the rituals that created them, each of these yuan-ti manifests their serpentine heritage in a variety of ways: a forked tongue, snake eyes, a snakelike nose, or some other ophidian characteristic. However a yuan-ti looks, they have the power to pursue great good or evil in the multiverse.