Monster Manual 5E

YUAN-TI
Yuan-ti are devious serpent folk devoid of compassion.
From remote temples in jungles, swamps, and deserts,
the yuan-ti plot to supplant and dominate all other races
and to make themselves gods. '
Forsaken Humanity. The yuan-ti were once humans
who thrived in the earliest days of civilization and
worshiped serpents as totem animals. They lauded the
serpent's sinuous flexibility, its calculated poise, and
its deadly strike. Their advanced philosophy taught
the virtue of detachment from emotion and of clear,
focused thought.
Yuan-ti culture was among the richest in the mortal
world. Their warriors were legendary, their empires
always expanding. Yuan-ti temples stood at the
centers of ancient metropolises, reaching ever higher
in prayer to the gods they longed to emulate. In time,
the serpent gods heard those prayers, their sibilant
voices responding from the darkness as they told the
yuan-ti what they must do. The yuan-ti religion grew
more fanatical in its devotion. Cults bound themselves
to the worship of the serpent gods and imitated their
ways, indulging in cannibalism and humanoid sacrifice.
Through foul sorcery, the yuan-ti bred with snakes,
utterly sacrificing their humanity to become like the
serpent gods in form, as well as in thought and emotion.
Serpent Kings of Fallen Empires. The yuan-ti view
their physical transformation as a transcendent moment
for their race, allowing them to shed their frail humanity
like dead skin. Those that did not transform eventually
became slaves or food for the blessed of the serpent
gods. The yuan-ti empires withered or were defeated by
those who fought against their cannibalism and slavery,
and the serpent folk were left in the ruins of their great
capitals, far removed from other races.
Cold of Heart. Humanoid emotions are foreign to
most yuan-ti, which understand sentiment only as an
exploitable weakness. A yuan-ti views the world and
the events of its own life with such extreme pragmatism
that it is nearly impossible to manipulate, influence, or
control by nonmagical means, even as it seeks to control
other creatures through terror, pleasure, and awe.
Yuan-ti know that the world they hope to rule can't be
bound for long by brute force, and that many creatures
will refuse to serve. As a result, yuan-ti first influence
other creatures with the promise of wealth and power.
Time and again, humanoid cultures make the fatal
mistake of trusting the yuan-ti. They forget that a yuan-ti
that acts honorably or lends aid in a time of trouble does
so only as part of a grander design.
Yuan-ti leaders are cunning and ruthless tacticians
who readily sacrifice lesser yuan-ti if potential victory
justifies such losses. They have no sense of honorable
combat and strike first in decisive ambush if they can.
False Worship. Yuan-ti life revolves around their
temples, yet yuan-ti don't love the gods they worship.
Instead, they see worship as a means to attain power. A
yuan-ti believes an individual who attains enough power
can devour and replace one of the yuan-ti gods. The
yuan-ti strive for ascension and are willing to commit
the darkest atrocities to achieve it.