Nagas - Serpents of the East

The Nagas (a Sanskrit word for Serpents) are powerful beings, meant to be able to appear as beautiful humans, or powerful, large cobra-like snakes. They are usually associated with water features (lakes, rivers, seas, and even wells). Brahma relegated them to the underground kingdom of Naga-Loka, sometimes called Patala-Loka, when they became to populous on Earth. This kingdom is filled with resplendent palaces filled with jewels. They are also commanded to bite only evil creatures, or those who would die prematurely, though they are as likely to kill any who would cross them. The Nagas also are tied to protecting treasures, and can be beneficial to humanity if treated with the proper respect. Female Nagas are called Naginis or Nagis, and are serpentine princesses of striking beauty. 
They are found in Hinduism, Buddhism & Jainism, primarily in India, and to some degree in the Far East. In Buddhism, especially in regions like Tibet they are seen as door guardians & as semi-divine. They are a major enemy of the Yuan-Ti, as the latter worships the worst aspects of the serpent, at least in deified form. Both have their followers, and regularly carry out covert wars against each other. The Yuan-Ti have found a process to prevent a Naga from regenerating after a number of days if killed, and raising the skeleton with limited abilities to serve them. This allows them to provoke the Nagas, and try to gain information & power over their enemies.
Naga society has two basic types of Naga, the Guardian Naga, and the Spirit Naga. The former is powerful and benevolent, helping other creatures where they can, protecting them from evil. If angered for any reason they will kill, and never forget who crossed them, even inflicting carnage on the descendants of their enemies. The latter, the Spirit Nagas, are evil through & through, and are highly paranoid, thinking that most others (including their own kind) are constantly plotting against them. They will happily enslave others and work with the followers of gods such as Kali, to dominate others.
All Nagas are effectively immortal and if killed will reappear after a few days back in Naga-Loka. There they will look to at least stop, if not outright kill, anyone who bested them. This is especially true for Spirit Nagas. The only way to truly stop a Naga this way is to kill it and wish that they don't regenerate. They breed like serpents and lay eggs in specific rooms in their palaces that act as nurseries.

Spread

The Nagas are found throughout much of the Far East from especially in regions with high amounts of Hindus and to a degree Buddhists. Most adherents to these believe in the Nagas being divine beings.

Variations & Mutation

Nagas can come in multiple types, some good, some evil, and one or two undead.

Cultural Reception

Most of the Far East have some type of snake worship in some degree. The Indian subcontinent seems to have encountered Nagas first, and have greatly influenced belief in them. It is likely the first were encountered in relation to the Ganges river, and helped cement it as a sacred river.
Many such bodies of water has now became linked to Nagas, including waterfalls, lakes, and of course rivers (at least spots in them).

In Literature

Some are mentioned in Sanskrit and Buddhist texts.

In Art

Many paintings, and wooden or stone carvings have took place and tied to religious Sanskrit or Buddhist sites. Some are seen as places of knowledge, or used for protection.