Asur - this word, including its several variants - Asurya, Asuryaa, and Aasur, is used in Rig Ved 88 times Derived from Asu, this word in Rig Ved is always used as an adjective meaning mighty. Although this adjective has always been used mostly for God, it has also been used for non-God entities. Towards the end of the Rig Vedik period, the use of this word underwent a drastic change; it became a noun and henceforth it was used for demons and hostile beings.

Iraanian emigrants still use it for God calling Him "Ahur Mazdaa" (corrupt of Sanskrit words Asur Madhiraa meaning the mighty and wise one). At one time this word was exclusively assigned to demons. The Sapt-Sindhu Aaarya coined the word "Sur" to denote gods, while continuing the use of the word "Dev" also to denote gods. In retaliation Iraanian Aarya started using the word "Dev" for devil.
(Aangiras, p 58)

The vilification of Asur happened in the later Vaidik period, when the Puraan were written. In the Rig Ved the word Asur does not have a negative connotation and many gods including Indra, Varun, Mitra, Agni etc have been described as Asur. Even Varun was called Asur and that may indicate some negative quality sometimes in some person but a person cannot be condemned because of that. Indra probably became the king of the gods as Varun did not qualify because he was having some Aasuree qualities.

Secondly we have the example of Prahlaad, who though a born as Asur was having non-Asur qualities. Mayaasur probably also had exalted status like that of Prahlaad. We also know that Bhagadatt from Pragjyotishpur (Kaamaroop) was also a dignified Asur king. Mayaasur, the great ancient Hindu astronomer, could have been from Pragjyotishpur.

Asur is Varun, the earliest and most prominent early Vaidik deity, as are other gods like Soorya, Chandra. The word Asur means "supreme spirit". Later Indra became most important. In the Pauraanik times the derivation changes to "A-Sur" - means who is not god - non-godly. "Sur", actually derived from "Asur", is a name of the Sun, besides meaning Som juice and one who propels (see Monier Williams Sanskrit Dictionary)

Dr Ram Vilas Sharma was a great Vaidik scholar who wrote in Hindi, though he knew English well. One of these books is about the spread of Rig Ved from India towards west. This book is written very well. In that book Sharma points out that practically all the Vaidik gods have been called Asur. For instance, according to him, Indra has been called Dev Indra Asur. Sharma explains the word 'Asur' as a derivative of 'Asu', meaning Praan/Vaayu, a symbol of power. According to him, the word Asur originally meant anyone with great power.

Asu = life; and ra = taker; thus Asur means life taker, or who takes life; while the word Sur means who takes Amrit or Divine.
(Valmiki Ramayana, 1/1/45)

There are over 6 different races of demons, Daitya having the most power and influence. They are known by the names of their mothers
who are as follows: Diti, Danu, Sinhikaa, Krodhaa, Danu and Kalaa.

(1) Daitya - Diti gave birth to the Daitya who are the most powerful since both Diti, the mother of the demons and Aditi who is the mother of the Devtaa (demigods) were close sisters. The Daitya are large and powerful beings with molten coppery complexions and dark colored hair. They are very adept at mystic powers and are at constant war with their cousins the Devtaa.

(2) Daanav - The Daanav are described as being more numerous yet slightly less powerful than the Daitya. They are sometimes described as being the color of clouds and sometimes having golden colored hair. They most resemble what might be called Giants or Titans as they are huge and strong bodied. they are also always at war with the gods.

(3) The son of Sinhikaa was a great and powerful serpent like demon named Raahu who was once a great demon lord until his head was cut off. Now he is in two pieces. The head is called Raahu and his tail is called Ketu.

(4) The sons of Krodhaa were all strong, war loving heroes, who took pleasure in battle, blood and killing.

(5) The sons of Danu were all vicious dragons the most powerful and famous being Vritraasur.

(6) The sons of Kalaa were all fierce and violent war lords.

Asur live primarily in the lower regions of the universe which are called Rasaatal, Sutal and Atal and there realms are without any Sun or
moonlight. The Naag Lok planet gives light to the planets that they live on. The cities of the demons stretch across the black and bleak horizon of the nether worlds. The bizarre reddish light, which reflects from the gems in the hoods of serpents of Naag Lok, spreads across the cites. Their walls are high and made of gold which glows reddish and bright in the low light, shimmering like the sunset on a windy sea. Various towers and buildings rise above the high outer defenses looking like the spikes on the back of some giant sleeping lizard. They are full of the various races of demons.

Within the walls of the cities are large parks and pleasure gardens in which the demons socialize. The Demons live for hundreds of millions of years and enjoy in pure passionate atheistic revelry. It is said that they drink large quantities of a drink made from milk and marijuana or different kinds of rare alcohols’ which make them feel invincible. They openly enjoy in vast parks with their women having no thought of shame. All the scenic environment is made of false plants made from gems and precious metals because no natural life will grow there.

The Guru of the demons is named Shukraachaarya and he is the Lord and master of the planet Venus. He is not evil, rather he is actually a good soul. He is so liberal that the demons came to him and asked him to be their priest and he agreed. There are whole groups of priests who work with the demons and do rituals for them. They teach them politics, atheistic philosophy and how to use Mantra weapons.

The constant conflict between the gods and the demons forms the main theme of most the Pauraanik histories. Demons such as Hiranyakashyap
and Hiranyaaksh are only a few of the most powerful demons mentioned. They envied the gods and were always at war with them. They were for the most part atheistic, occasionally however they worship certain deities such as Shiv or Brahmaa for favors or boons so they could defeat their enemies.

Asur will form pacts with Raakshas if it seems in their favor and utilize the help of Naag but will generally have nothing to do with any other species. They have a love/hate relationship with humans. They will use humans if it fits their need but they have no trouble killing them if they get in the way. They are always fighting with the gods over the control of earth. They have a general hatred for all natural life and everything that most people would call “good” and will stop at nothing to pollute Nature and the minds of humanity.